Koji Mitsui

is a Japanese retired professional baseball pitcher. He played in Nippon Professional Baseball for the Seibu / Saitama Seibu Lions from 2001 through 2009.

Career Mitsui was drafted in the 4th round of the 1991 NPB draft by the Fukuoka SoftBank Hawks, but opted not to sign. He pitched for Nippon Steel Hirohata for most of the decade. In 2000, the Seibu Lions took him in the 2nd round. He debuted in 2001, going 3–3 with a 5.22 ERA as a swingman. He allowed one run in innings in the 2008 Japan Series, which Seibu won.

Mitsui requested to be posted by the Lions to Major League Baseball twice in the 2008–09 offseason, but attracted no bids on either occasion.