Isao Ijima (飯島魁, Iijma Isao, July 24, 1861 – March 14, 1921) was a Japanese zoologist known for his studies of sponges (Porifera) — including his circumscription of the genus Staurocalyptus — leeches (Hirudinea), flatworms (Turbellaria), birds, and fish.

[4][5] Born in Hamamatsu in 1861 into a samurai family of Hamamatsu Domain, at the age of fifteen he entered the Kaisei Gakkō [ja] school in Tokyo, before enrolling as a student in the Science College at the Imperial University, Tokyo in 1878.

[2] In 1879, together with Sasaki Chūjirō [ja], both having previously received training from and assisted Morse in his exploration of the Ōmori Shell Mounds, Ijima excavated the Okadaira Shell Mound; this is credited with being the first modern archaeological survey conducted solely by Japanese.

[1] Returning to Japan in 1886, at the age of 25 he was appointed Professor of Zoology at the Imperial University, Tokyo, where he remained until his death.

[2][12] In his personal life, Ijima enjoyed hunting, shooting, fishing, wine, and smoking a pipe.