Kōji Koseki is a Japanese manga artist known for his work in the 1970s and 1980s, particularly within the sports genre. His career includes serializations in Weekly Shōnen Jump, one of Japan's most prominent manga magazines.

Koseki authored the manga Panku Tōshu, which was published in Weekly Shōnen Jump in 1975. The series appeared in the 3rd-4th combined issue and continued through issue 15 of that year.

He is also the creator of the baseball manga Kenritsu Umisora Kōkō Yakyū Buin Yamashita Tarō-kun. This work was adapted into an original video animation (OVA) released on September 23, 1988. The OVA, produced as a single episode with a runtime of approximately 34 minutes, tells the story of Tarō Yamashita, a passionate but unskilled baseball player who works to improve and become a key player for his high school team, Umisora High.