Mitsui (written: 三井 lit. "three wells" or 光井) is a Japanese surname. Notable people with the surname include:

• Aika Mitsui (光井 愛佳, born 1993), Japanese singer
• Airi Mitsui (三井 愛梨, born 2004), Japanese swimmer
• Hideki Mitsui (三井 秀樹, born 1963), Japanese screenwriter
• Kōji Mitsui (三井 弘次, 1910–1979), Japanese actor
• Miyoko Mitsui (三井 美代子, born 1919), Japanese hurdler
• Risako Mitsui (三井 梨紗子, born 1993), Japanese synchronized swimmer
• Takaharu Mitsui (三井 高陽, 1900–1983), Japanese philatelist
• Mitsui Takatoshi (三井 高利, 1622–1694), founder of the Mitsui conglomerate
• Takuo Mitsui (三井 卓雄, 1909–?), Japanese sailor
• Wakio Mitsui (三井 辨雄, 1942–2021), Japanese politician
• Yōko Mitsui (三井 葉子, 1936–2014), Japanese poet
• Yuri Mitsui (三井 ゆり, born 1968), Japanese model and actress