koji or kōji may refer to:
- Koji, the common name of the fungus Aspergillus oryzae
- Koji Ishikawa (illustrator) (いしかわこうじ, born 1963), picture-book author and illustrator
- Koji Kondo (born 1960), composer of video game music
- Koji Minamoto, character from Digimon Frontier
- Koji Sawai (born 1952), anime director
- Koji or Kōji Seki, multiple people
- Kōji Seto (born 1988), Japanese actor
- Koji Shimizu (born 1969), long-distance runner
- Kōji Tamaki (born 1958), lead singer of the band Anzen Chitai (安全地帯)
- Kōji Totani (1948–2006), voice actor
- Koji (Ōban), a character from the series Ōban Star-Racers
In Japan: Kōji may refer to:
- Kōji (Heian period) (康治), Japanese era, 1142–1144
- Kōji (Muromachi period) (弘治), Japanese era, 1555–1558