Hiroki Kosai (香西 洋樹 Kōsai Hiroki, born 8 February 1933) is a Japanese astronomer with the Tokyo Astronomical Observatory. He is best known for helping to popularize astronomy in Japan and for his observations of comets and asteroids, most notably with his co-discovery the comet 1976 XVI. The asteroid 3370 Kohsai is named in his honor.[1]