It can refer to a number of people, including the following members of the prominent Koizumi family:

• Junichiro Koizumi, former prime minister of Japan
• Junya Koizumi, a second-generation Diet member and the father of Junichiro Koizumi
• Kotaro Koizumi, an aspiring actor and the first son of Junichiro Koizumi
• Matajiro Koizumi, the grandfather of Junichiro Koizumi

Other individuals include:

• Eiko Koizumi, a Japanese beach volleyball player
• Gunji Koizumi, the founder of British Judo
• Kyoko Koizumi, a singer and actress
• Takashi Koizumi, a film director
• Patrick Lafcadio Hearn, a.k.a. Yakumo Koizumi

Koizumi Station is either

• Koizumi Station (Ehime) (古泉駅)
• Koizumi Station (Gifu) (小泉駅)

It can refer to a number of fictional characters, including:

• Akako Koizumi, character in Magic Kaito
• Mika Koizumi, character in Choudenshi Bioman
• Itsuki Koizumi, character in Suzumiya Haruhi no Yuuutsu otherwise known to most American fans as the anime, The Melancholy of Haruhi Suzumiya.