Koiwai (written: 小岩井 lit "small rock well") is a Japanese surname. Notable people with the surname include:
Eichi Karl Koiwai (1920–2009), American judoka , Japanese voice actress , Japanese figure skater Sakura Koiwai (小祝さくら, born 1998), Japanese golfer
Fictional characters , titular character of the manga series Yotsuba&!
Yoshino Koiwai, character from Masamune-kun's No Revenge series
See also Koiwai Station, a railway station in Takizawa, Iwate Prefecture, Japan Kowiai language, also spelled Koiwai