Iwai (written: 岩井 lit. "rock well" or 岩居) is a Japanese surname. Notable people with the surname include:
- Akira Iwai (岩井 章, 1922–1997), Japanese trade union leader
- Atsuhiro Iwai (岩井 厚裕, born 1967), Japanese footballer
- Fumio Iwai (岩井 文男, born 1960), Japanese politician
- Fumito Iwai (岩井 郁人, born 1990), Japanese musician
- Iwai Hanshirō I (1652–1699), Japanese kabuki artist
- Iwai Hanshirō V (1776–1847), Japanese kabuki artist
- Iwai Hanshirō VIII (1829–1882), Japanese kabuki artist
- Katsuhito Iwai (岩井 克人, born 1947), Japanese economist
- Masaki Iwai (岩井 真幸, born 1985), Japanese curler
- Shigeki Iwai (岩井 茂樹, born 1968), Japanese politician
- Shunji Iwai (岩井 俊二, born 1963), Japanese film director
- Shuya Iwai (岩井 柊弥, born 2000), Japanese footballer
- Toshio Iwai (岩井 俊雄, born 1962), Japanese artist
- Yukiko Iwai (voice actress) (岩居 由希子, born 1972), Japanese voice actress
- Yukiko Iwai (Onyanko Club) (岩井 由紀子, born 1968), Japanese pop singer