Koizumi (小泉 "small fountain", 古泉 "old fountain") is a Japanese family name.
It may describe one of two Koizumi Stations.
It can refer to a number of people, including the following members of the prominent Koizumi family:
- Junichirō Koizumi (小泉 純一郎, born 1942), former prime minister of Japan
- Junya Koizumi (小泉 純也, 1904–1969), a second-generation Diet member and the father of Junichiro Koizumi
- Matajirō Koizumi (小泉 又次郎, 1865–1951), Japanese politician and the father-in-law of Junya Koizumi
- Kōtarō Koizumi (小泉 孝太郎, born 1978), an aspiring actor and the first son of Junichiro Koizumi
- Shinjirō Koizumi (小泉 進次郎, born 1981), a fourth-generation Diet member and the second son of Junichiro Koizumi
- Eiko Koizumi (小泉 栄子, born 1973), a Japanese beach volleyball player
- Gunji Koizumi (小泉 軍治, 1885–1965), the founder of British Judo
- Kyōko Koizumi (小泉 今日子, born 1966), singer and actress
- Takashi Koizumi (小泉 堯史, born 1944), film director
- Patrick Lafcadio Hearn or Yakumo Koizumi (小泉 八雲, 1850–1904), Irish-Greek-Japanese author
- Yoshiaki Koizumi (小泉 歓晃, born 1968), video game designer
- Akako Koizumi, character in Magic Kaito
- Asami Koizumi from the Young Justice animated series
- Itsuki Koizumi (古泉 樹), character in Haruhi Suzumiya series
- Mika Koizumi, character in Choudenshi Bioman
- Risa Koizumi, the protagonist of Love Com
- Several characters in Guru Guru Pon-chan, Ponta, Yuki, Soichiro, and Ji Koizumi
- Mahiru Koizumi, a character in "Danganronpa 2 : Goodbye Despair"
- Hanayo Koizumi, a character from "Love Live!"
- Gen-ichi Koidzumi (小泉 源一, 1883-1953), botanist whose standard abbreviation in botanical works is Koidz.