1931 (MCMXXXI) was a common year starting on Thursday (dominical letter D) of the Gregorian calendar, the 1931st year of the Common Era (CE) and Anno Domini (AD) designations, the 931st year of the 2nd millennium, the 31st year of the 20th century, and the 2nd year of the 1930s decade.
- January – The National Committee for Modification of the Volstead Act is formed to work for the repeal of Prohibition in the United States.
- January 2 – South Dakota native Ernest Lawrence invents the cyclotron, used to accelerate particles to study nuclear physics.
- January 3 – Albert Einstein begins doing research at the California Institute of Technology, along with astronomer Edwin Hubble.
- January 4 – German pilot Elly Beinhorn begins her flight to Africa.
- January 6 – Thomas Edison submits his last patent application.
- January 22 – Sir Isaac Isaacs is sworn in as the first Australian-born Governor-General of Australia.
- January 25 – Mohandas Gandhi is again released from imprisonment in India.
- January 27 – Pierre Laval forms a government in France.
- January 30 – Release of the movie City Lights starring Charlie Chaplin.
- February 3 – Hawke's Bay earthquake: Much of the New Zealand cities of Napier and Hastings are destroyed in an earthquake measuring 7.9 on the Richter scale killing 256 people.
- February 4 – Soviet leader Joseph Stalin gives speech calling for rapid industrialization, arguing that only strong industrialized countries will win wars while "weak" nations are "beaten". Stalin states : "We are fifty or a hundred years behind the advanced countries. We must make good this distance in ten years. Either we do it, or they will crush us." Intensification of the First Five-Year Plan in the Soviet Union for industrialization and collectivization of agriculture.
- February 10 – Official inauguration ceremonies for New Delhi as the capital of India begin.
- February 11 – National Socialist (NSDAP) and German National People's Party (DNVP) members walk out of the German Reichstag in protest against changes in the parliament's protocol intended to limit heckling.
- February 12 – Vatican Radio first broadcasts.
- February 14 – The original film version of Dracula with Bela Lugosi is released in the United States.
- February 16 – Pehr Evind Svinhufvud is elected president of Finland.
- February 20 – California gets the go-ahead by the United States Congress to build the San Francisco–Oakland Bay Bridge.
- February 21 – Peruvian revolutionaries hijack a Ford Trimotor aeroplane and demand that the pilot drop propaganda leaflets over Lima.
- March 1 – The USS Arizona is placed back in full commission after a refit.
- March 1 – Sir Oswald Mosley founds the New Party as a breakaway from the Labour Party in the United Kingdom.
- March 3 – The Star-Spangled Banner is adopted as the United States' National anthem.
- March 5 – The British viceroy of India and Mohandas Gandhi sign the Gandhi–Irwin Pact.
- March 7 – The new House of Representatives opens in Helsinki, Finland.
- March 11 – The Ready for Labour and Defence of the USSR programme, abbreviated as GTO, is introduced in the Soviet Union.
- March 17 – Nevada legalizes gambling.
- March 19 – Westminster St George's by-election in the U.K. results in the victory of the Conservative candidate Duff Cooper. The by-election has been treated virtually as a referendum on the leadership of the Conservative leader Stanley Baldwin, and Duff Cooper's victory ends the campaign by the press barons Lord Beaverbrook and Viscount Rothermere to oust Baldwin.
- March 23 – Indian revolutionary leaders Bhagat Singh, Shivaram Rajguru and Sukhdev Thapar are hanged for conspiracy to murder in the British Raj.
- March 25 – The Scottsboro Boys are arrested in Alabama and charged with rape.
- March 27 – English writer Arnold Bennett dies of typhoid in London shortly after returning from a visit to Paris, where he drank local water to prove it was safe.
- March 31 – An earthquake destroys Managua, Nicaragua, killing 2,000 people.
- April 1 – The Second Encirclement Campaign against Jiangxi Soviet in China is launched by the Kuomintang government to destroy the Communist forces in Jiangxi province.
- April 6 – The Portuguese government declares martial law in Madeira and in the Azores because of an attempted military takeover in Funchal.
- April 9 – Argentinian anarchist Severino Digiovanni is executed.
- April 12 – Municipal elections in Spain, which are treated as a virtual referendum on the monarchy, result in the triumph for the republican parties.
- April 14 – The Second Spanish Republic is proclaimed in Madrid. Meanwhile, as a result of the victory of the Republican Left of Catalonia, Francesc Macià proclaimed in Barcelona the Catalan Republic.
- April 15 – The Castellammarese War ends with the assassination of Joe "The Boss" Masseria, briefly leaving Salvatore Maranzano as capo di tutti i capi ("boss of all bosses") and undisputed ruler of the American Mafia. Maranzano is himself assassinated less than 6 months later, leading to the establishment of the Five Families.
- April 17 – After the negotiations between the republican ministers of Spain and Catalonia, the Catalan Republic transforms itself into the Generalitat de Catalunya, an autonomous government inside the Spanish Republic.
- April 22 – Austria, the UK, Denmark, Germany, Italy, Sweden and the USA recognize the Spanish Republic.
- April 25 – The automobile manufacturer Porsche is founded by Ferdinand Porsche in Stuttgart.
- May 1 – Construction of the Empire State Building is completed in New York City.
- May 4 – Kemal Atatürk is re-elected president of Turkey.
- May 5 – İsmet İnönü forms a new government in Turkey (7th government).
- May 11 – The Creditanstalt, Austria's largest bank, goes bankrupt, beginning the banking collapse in Central Europe that causes a worldwide financial meltdown.
- May 13 – Paul Doumer is elected president of France.
- May 14 – Ådalen shootings: Five people are killed in Ådalen, Sweden, when soldiers open fire on an unarmed trade union demonstration.
- May 15
- May 31 – The Second Encirclement Campaign against Jiangxi Soviet ends in defeat of the Kuomintang.
- June 3 – Salvador Dalí's The Persistence of Memory is put on display for the first time in Paris at the Galerie Pierre Colle.
- June 5 – German Chancellor Dr. Heinrich Brüning visits London, where he warns the British Prime Minister Ramsay MacDonald that the collapse of the Austrian banking system, caused by the bankruptcy of the Creditanstalt, has left the entire German banking system on the verge of collapse.
- June 12 – English cricketer Charlie Parker equals J. T. Hearne's record for the earliest date to reach 100 wickets.
- June 14 – Saint-Philibert disaster: the overloaded pleasure craft Saint-Philibert, carrying trippers home to Nantes from the Île de Noirmoutier, sinks at the mouth of the river Loire in France; over 450 drown.
- June 19
- June 23–July 1 – Wiley Post and Harold Gatty accomplish the first round-the-world flight in a single-engine plane, flying eastabout from Roosevelt Field, New York, in 8 days, 15 hours, 51 minutes.
- July – John Haven Emerson of Cambridge, Massachusetts perfects his negative pressure ventilator ("iron lung") just in time for the growing polio epidemic.
- July 1 – Rebuilt Milano Centrale railway station officially opens in Italy.
- July 9 – Irish racing driver Kaye Don breaks the world water speed record at Lake Garda, Italy.
- July 13 – Royal soldiers shoot and kill 22 people demonstrating against the Maharaja Hari Singh of the Indian princely state of Kashmir and Jammu.
- July 16 – Emperor Haile Selassie signs the first constitution of Ethiopia.
- July 26 – The millennialist Bible Student movement adopts the name Jehovah's Witnesses at a meeting in Columbus, Ohio.
- July 31 – The May Report in the United Kingdom recommends extensive cuts to government expenditure. This produces a political crisis as many members of the Labour Party (at this time in government) object to the proposals.
- The 1931 China floods reach their peak in possibly the deadliest natural disaster yet recorded.
- Warner Brothers releases the first Merrie Melodies cartoon, Lady, Play Your Mandolin.
- August 9 – Referendum in Prussia for dissolving the Landtag ends with the "yes" side winning 37% of the vote, which is insufficient for calling the early elections. The elections are intended to remove the Social Democratic Party (SPD) government of Otto Braun, which is one of the strongest forces for democracy in Germany. Supporting the "yes" side were the NSDAP, the DNVP and the Communist Party (KPD) while supporting the "no" side were the SPD and Zentrum.
- August 11 – Run on the British pound leads to political and economic crisis in Britain.
- August 24 – The Labour Government of Ramsay MacDonald resigns in Britain, replaced by a National Government of people drawn from all parties, also under MacDonald.
- September 5 – John Thomson, Scottish football player, dies as the result of an accident during a Celtic–Rangers match.
- September 7 – Second Round Table Conference on the constitutional future of India opens in London. Mahatma Gandhi represents the Indian National Congress.
- September 10 – The worst hurricane in British Honduras history kills an estimated 1,500.
- September 15 – Invergordon Mutiny: Strikes are called in the British Royal Navy due to decreased pay.
- September 16 – Hanging of resistance leader Omar Mukhtar in Italian Libya.
- September 18
- September 20 – With a gun literally pointed to his head the Chinese commander of Kirin province announces the annexation of that territory to Japan.
- September 22 – The United Kingdom abandons the gold standard.
- October – The Caltech Department of Physics faculty and graduate students meet with Albert Einstein as a guest.
- October 4 – Dick Tracy, the comic strip detective character created by cartoonist Chester Gould, makes his debut appearance in the Detroit Mirror newspaper.
- October 5 – American aviators Clyde Edward Pangborn and Hugh Herndon, Jr., complete the first non-stop flight across the Pacific Ocean, from Misawa, Japan, to East Wenatchee, Washington, in 41½ hours.
- October 11 – Rally in Bad Harzburg, Germany leads to the Harzburg Front being founded, uniting the NSDAP, the DNVP, the Stahlhelm and various other right-wing fractions.
- October 17
- October 24 – The George Washington Bridge across the Hudson River in the United States is dedicated; it opens to traffic the following day. At 3,500 feet (1,100 m), it nearly doubles the previous record for the longest main span in the world.
- October 27 – United Kingdom general election results in the victory of the National Government and the defeat of Labour Party in the country's greatest ever electoral landslide.
- November 7 – The Chinese Soviet Republic is proclaimed by Mao Zedong.
- November 8
- November 21 – The infamous Red-and-White Party, given by Arthur Jeffress in Maud Allan’s Regent's Park townhouse in London, marks the end of the "Bright young things" subculture in Britain.
- November 25
- December 5 – Original Cathedral of Christ the Saviour in Moscow (1883) is dynamited by order of Joseph Stalin.
- December 8 – Carl Friedrich Goerdeler is appointed Reich Price Commissioner in Germany to enforce the deflationary policies of the Brüning government.
- December 9 – The Spanish Constituent Cortes approves the Spanish Constitution of 1931, effectively establishing the Second Spanish Republic.
- December 10
- December 11 – The Parliament of the United Kingdom enacts the Statute of Westminster, which establishes a status of legislative equality between the self-governing dominions of the Commonwealth of Australia, the Dominion of Canada, the Irish Free State, Newfoundland, the Dominion of New Zealand and the Union of South Africa.
- December 13 – Wakatsuki Reijirō resigns as Prime Minister of Japan.
- December 26 – Phi Iota Alpha, the oldest surviving Latino fraternity, is founded in the United States.
- Ust-Abakanskoye becomes Abakan.
- January 2 – Toshiki Kaifu, Prime Minister of Japan
- January 5
- January 6
- January 8 – Bill Graham, German concert promoter (d. 1991)
- January 10 – Peter Barnes, English playwright and screenwriter (d. 2004)
- January 12 – Roland Alphonso, Jamaican musician (d. 1998)
- January 13 – Charles Nelson Reilly, American actor (d. 2007)
- January 14 – Caterina Valente, French singer and actress
- January 16
- January 17 – James Earl Jones, American actor
- January 18 – Chun Doo-hwan, President of South Korea
- January 19
- January 20 – David Lee, American physicist, recipient of the Nobel Prize in Physics
- January 22 – Sam Cooke, American singer (d. 1964)
- January 24 – Lars Hörmander, Swedish mathematician (d. 2012)
- January 25 – Dean Jones, American actor (d. 2015)
- January 26 – Alfred Lynch, English actor (d. 2003)
- January 27 – Mordecai Richler, Canadian author (d. 2001)
- January 29 – Ferenc Mádl, President of Hungary (d. 2011)
- January 30 – Allan W. Eckert, American historian, naturalist, and author (d. 2011)
- January 31 – Ernie Banks, American baseball player (d. 2015)
- February 1
- February 2
- February 6
- February 8 – James Dean, American actor (d. 1955)
- February 9
- February 11 – Larry Merchant, American author and boxing commentator
- February 13 – Geoff Edwards, American actor and game show host (d. 2014)
- February 15 – Claire Bloom, English actress
- February 16
- February 18
- February 24 – Brian Close, English cricketer
- February 25 – Eric Edgar Cooke, Australian serial killer (d. 1964)
- February 28
- March 2
- March 3 – John Smith, American actor (d. 1995)
- March 4
- March 8 – Neil Postman, American media theorist and cultural critic (d. 2003)
- March 9 – Jackie Healy-Rae, Irish Politician (d. 2014)
- March 11
- March 12 – Herb Kelleher, American businessman
- March 15 – Ted Marchibroda, American football player (d. 2016)
- March 18
- March 20
- March 22
- March 24 – Connie Hines, American actress (d. 2009)
- March 26 – Leonard Nimoy, American actor and director (d. 2015)
- March 27 – David Janssen, American actor (d. 1980)
- March 29
- April 1
- April 5 – Héctor Olivera, Argentine film director, producer and screenwriter
- April 6 – Suchitra Sen (Roma Dasgupta), legendary Bengali actress (d. 2014)
- April 11
- April 15
- April 27 – Igor Oistrakh, Ukrainian violinist
- April 29
- May 6 – Willie Mays, African-American baseball player
- May 7 – Teresa Brewer, American pop and jazz singer (d. 2007)
- May 10 – Ichirō Nagai, Japanese voice actor (d. 2014)
- May 13
- May 14 – Alvin Lucier, American composer
- May 15
- May 16
- May 17 – Marshall Applewhite, Heaven's Gate religious sect founder (d. 1997)
- May 18
- May 19 – Éric Tappy, Swiss tenor
- May 20 – Ken Boyer, American baseball player (d. 1982)
- May 23
- May 25 – Georgy Grechko, Russian cosmonaut
- May 27 – Faten Hamama, Egyptian actress (d. 2015)
- May 28 – Carroll Baker, American actress
- May 30 – Father John O'Brien, Irish priest and musician (d. 2008)
- May 31
- June 2 – Larry Jackson, American baseball player (d. 1990)
- June 3
- June 7 – Malcolm Morley, English-born painter
- June 8 – Dana Wynter, German-born American actress (d. 2011)
- June 9
- June 10 – João Gilberto, Brazilian musician
- June 13 – Moysés Baumstein, Brazilian holographer and artist (d. 1991)
- June 14
- June 18 – Fernando Henrique Cardoso, President of Brazil
- June 20
- June 21 – Margaret Heckler, American Secretary of Health and Human Services
- June 24 – Billy Casper, American golfer
- June 25 – Vishwanath Pratap Singh, Prime Minister of India (d. 2008)
- June 26 – Colin Wilson, British writer (d. 2013)
- June 27 – Martinus J. G. Veltman, Dutch physicist, recipient of the Nobel Prize in Physics
- June 29 – Brian Hutton, Baron Hutton, Lord Chief Justice of Northern Ireland and British Lord of Appeal in Ordinary
- July 1
- July 2 – Robert Ito, Canadian actor
- July 4 – Stephen Boyd, Irish actor (d. 1977)
- July 5 – Ismail Mahomed, South African and Namibian Chief Justice (d. 2000)
- July 6
- July 7 – David Eddings, American novelist (d. 2009)
- July 10
- July 11 – Tab Hunter, American actor and singer (Damn Yankees)
- July 15 – Clive Cussler, American author
- July 19 – Mary Lou Studnicka, American female professional baseball player
- July 23 – Te Arikinui Dame Te Atairangikaahu, Māori queen (d. 2006)
- July 26 – Fred Foster, American songwriter and record producer
- July 27 – Jerry Van Dyke, American comedian and actor
- August 2 – Ruth Maria Kubitschek, German actress
- August 7 – Charles E. "Charlie" Rice, American legal scholar and author
- August 9 – Mário Zagallo, Brazilian football player and manager
- August 10 – Tom Laughlin, American actor (Billy Jack) (d. 2013)
- August 12 – William Goldman, American author
- August 15
- August 16 – Marion Patrick Jones, Trinidadian writer (d. 2016)
- August 18
- August 19 – Willie Shoemaker, American jockey (d. 2003)
- August 20 – Don King, American boxing promoter
- August 23
- August 25
- August 27
- August 28 – John Shirley-Quirk, English bass-baritone (d. 2014)
- August 30
- August 31
- September 1
- September 4 – Mitzi Gaynor, American actress, singer and dancer
- September 8 – Jack Rosenthal, English playwright (d. 2004)
- September 10 – Philip Baker Hall, American actor
- September 12
- September 17 – Anne Bancroft, American actress (d. 2005)
- September 21
- September 22
- September 23 – Gerald Merrithew, Canadian educator and statesman (d. 2004)
- September 27 – Freddy Quinn, Austrian singer and actor
- September 29
- September 30
- October 1 – Alan Wagner, American opera critic (d. 2007)
- October 2 – Morris Cerullo, American televangelist
- October 3 – Denise Scott Brown, American architect
- October 6 – Riccardo Giacconi, Italian-born physicist, recipient of the Nobel Prize in Physics
- October 7
- October 13 – Eddie Mathews, baseball player (d. 2001)
- October 15 – Avul Pakir Jainulabdeen Abdul Kalam, President of India (d. 2015)
- October 16
- October 17
- October 19
- October 20 – Mickey Mantle, American baseball player (d. 1995)
- October 22 – Ann Rule, American true-crime writer (d. 2015)
- October 23
- October 25 – Jimmy McIlroy, Irish footballer and football manager
- October 26 – Hank Garrett, American actor and comedian
- October 28 – Harold Battiste, American composer and arranger (d. 2015)
- October 31 – Dan Rather, American television news reporter
- November 2 – Phil Woods, American saxophonist (d. 2015)
- November 3
- November 4 – Marie Mansfield, American professional baseball player
- November 5 – Ike Turner, African-American singer and songwriter (d. 2007)
- November 6 – Mike Nichols, German-American television actor, writer and director (d. 2014)
- November 8
- November 9 – Whitey Herzog, American baseball player
- November 12 – Mary Louise Wilson, American actress and singer
- November 15 – Mwai Kibaki, third President of Kenya
- November 16 – Hubert Sumlin, American blues musician (d. 2011)
- November 21
- November 26 – Adolfo Pérez Esquivel, Argentine activist, recipient of the Nobel Peace Prize
- November 28
- November 29 – Shintaro Katsu, Japanese actor (d. 1997)
- December 1
- December 2
- December 3 – Jaye P. Morgan, American singer, chanteuse
- December 9 – Ladislav Smoljak, Czech film and theater director, actor and screenwriter (d. 2010)
- December 11 – Rita Moreno, Puerto Rican-American actress
- December 12 – Lionel Blair, British actor, choreographer, dancer, headmaster and TV presenter
- December 15 – Klaus Rifbjerg, Danish writer (d. 2015)
- December 17 – Dave Madden, Canadian American actor (d. 2014)
- December 23 – Ronnie Schell, American actor
- December 24 – Mauricio Kagel, Argentine composer (d. 2008)
- December 27 – Edward E. Hammer, American electrical engineer and inventor (d. 2012)
- December 28 – Martin Milner, American actor (d. 2015)
- December 30 – Skeeter Davis, American singer (d. 2004)
- December 31 – Bob Shaw, Irish writer (d. 1996)
- January 3 – Joseph Joffre, French World War I general (b. 1852)
- January 4
- January 11 – James Milton Carroll, Baptist pastor, historian, and author (b. 1852)
- January 14 – Hardy Richardson, American baseball player (b. 1855)
- January 22 – Alma Rubens, American actress (b. 1897)
- January 23
- February 11 – Charles Algernon Parsons, British inventor (b. 1854)
- February 16 – Wilhelm von Gloeden, German photographer (b. 1856)
- February 18 – Louis Wolheim, American actor (b. 1880)
- February 23
- February 26 – Otto Wallach, German chemist, Nobel Prize laureate (b. 1847)
- February 28 – Thomas S. Rodgers, American admiral (b. 1858)
- March 5 – Arthur Tooth, Anglican clergyman (b. 1839)
- March 7
- March 11 – F. W. Murnau, German director (b. 1888)
- March 20
- March 21 – Bhagat Singh, Indian revolutionary (b. 1908)
- March 24 – Robert Edeson, American actor (b. 1868)
- March 25 – Ida Wells, African-American lynching crusader.
- March 27 – Arnold Bennett, English novelist (b. 1867)
- March 28 – Ban Johnson, American baseball executive (b. 1864)
- March 31 – Knute Rockne, American football coach (b. 1888)
- April 8 – Erik Axel Karlfeldt, Swedish writer, Nobel Prize laureate (b. 1864)
- April 9 – Nicholas Longworth, American politician, Speaker of the House (b. 1869)
- April 10 – Khalil Gibran, Lebanese poet and painter (b. 1883)
- April 14 – Richard Armstedt, German historian (b. 1851)
- April 15 – Joe Masseria, Italian-born American gangster (b. 1886)
- April 20 – Sir Cosmo Duff-Gordon, British baronet and Titanic survivor (b. 1862)
- April 26 – George Herbert Mead, American philosopher, sociologist and psychologist (b. 1863)
- April 30 – Sammy Woods, English cricketer (b. 1867)
- May 2 – George Fisher Baker, American financier and philanthropist (b. 1840)
- May 9 – Albert Abraham Michelson, German-born physicist, Nobel Prize laureate (b. 1852)
- May 14 – David Belasco, American Broadway impresario, theater owner and playwright (b. 1853)
- June 2 – Joseph Farnham, American screenwriter (b. 1884)
- June 4 – Hussein bin Ali, Sharif of Mecca, Arab nationalist
- June 8 – Virginia Frances Sterrett, American artist and illustrator (b. 1900)
- July 4
- July 12 – Nathan Söderblom, Swedish archbishop, recipient of the Nobel Peace Prize (b. 1866)
- August 6 – Bix Beiderbecke, American jazz trumpeter (b. 1903)
- August 15 – Nigar Shikhlinskaya, Azerbaijani World War I nurse (b. ca. 1878)
- August 26
- August 27 – Francis Marion Smith, American businessman (b. 1846)
- September 5 – John Thomson, Scottish footballer (b. 1909)
- September 10 – Salvatore Maranzano, Italian-American mobster (b. 1886)
- September 12
- September 16 – Omar Mukhtar, the leader of Libyan resistance (b. 1858)
- September 17
- September 18 – Geli Raubal, Hitler's niece (b. 1908)
- September 19 – David Starr Jordan, American ichthyologist, educator, eugenicist, and peace activist (b. 1851)
- October 3 – Carl Nielsen, Danish composer (b. 1865)
- October 13 – Ernst Didring, Swedish writer (b. 1868)
- October 18 – Thomas Edison, American inventor (b. 1847)
- October 21 – Arthur Schnitzler, Austrian author and dramatist (b. 1862)
- October 24 – Sir Murray Bisset, South African cricketer and Governor of Southern Rhodesia (b. 1876)
- November 4 – Buddy Bolden, African-American musician (b. 1877)
- November 6 – Jack Chesbro, American baseball player and MLB Hall of Famer (b. 1874)
- November 11 – Shibusawa Eiichi, Japanese industrialist (b. 1840)
- November 13 – Ivan Fichev, Bulgarian general, minister of defense, military historian, and academician (b. 1860)
- November 20 – Julius Drewe, English businessman, retailer and entrepreneur (b. 1856)
- November 21 – Bruno von Mudra, German general (b. 1851)
- December 2 – Vincent d'Indy, French composer (b. 1851)
- December 5 – Vachel Lindsay, American poet (b. 1879)
- December 9 – Antonio Salandra, Italian statesman, former Prime Minister (b. 1853)
- December 18 – Jack Diamond, American gangster (b. 1897)
- December 23 – Tyrone Power, Sr., American actor (b. 1869)
- December 26 – Melvil Dewey, American librarian, inventor of Dewey Decimal Classification (b. 1851)
- December 27 – José Figueroa Alcorta, 16th President of Argentina (b. 1860)