| Hiroki Kosai | |
| Native Name: | 香西 洋樹 |
| Native Name Lang: | ja |
| Birth Date: | 2 August 1933 |
| Nationality: | Japanese |
| Fields: | Astronomy |
| Workplaces: | Tokyo Astronomical Observatory |
| Thesis1 Title: | and |
| Thesis2 Title: | )--> |
| Thesis1 Url: | and |
| Thesis2 Url: | )--> |
| Thesis1 Year: | and |
| Thesis2 Year: | )--> |
| Doctoral Advisors: | )--> |
| Known For: | Helping to popularize astronomy in Japan |
| Spouses: | )--> |
| Partners: | )--> |
| see |
is a Japanese astronomer with the Kiso 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. He is credited by the Minor Planet Center (MPC) with the discovery of nearly 100 asteroids.
Asteroid 3370 Kohsai, discovered by Karl Reinmuth at Heidelberg Observatory in 1930, is named in his honor. The official naming citation was published by the MPC on 14 April 1987 .
Hiroki Kosai discovered several asteroids, all in collaboration with Kiichirō Furukawa and Goro Sasaki:
| 2271 Kiso | 22 October 1976 |   | |
| 2330 Ontake | 18 February 1977 |   | |
| 2470 Agematsu | 22 October 1976 |   | |
| 2924 Mitake-mura | 18 February 1977 |   | |
| 2960 Ohtaki | 18 February 1977 |   | |
| 3111 Misuzu | 19 February 1977 |   | |
| 3249 Musashino | 18 February 1977 |   | |
| 3291 Dunlap | 14 November 1982 |   | |
| 3319 Kibi | 12 March 1977 |   | |
| 3320 Namba | 14 November 1982 |   | |
| 3391 Sinon | 18 February 1977 |   | |
| 3392 Setouchi | 17 December 1979 |   | |
| 3607 Naniwa | 18 February 1977 |   | |
| 3878 Jyoumon | 14 November 1982 |   | |
| 4072 Yayoi | 30 October 1981 |   | |
| 4077 Asuka | 13 December 1982 |   | |
| 4186 Tamashima | 18 February 1977 |   | |
| 4272 Entsuji | 12 March 1977 |   | |
| 4526 Konko | 22 May 1982 |   | |
| 4812 Hakuhou | 18 February 1977 |   |
| 4855 Tenpyou | 14 November 1982 |   | |
| 4890 Shikanosima | 14 November 1982 |   | |
| 4929 Yamatai | 13 December 1982 |   | |
| 4963 Kanroku | 18 February 1977 |   | |
| 5017 Tenchi | 18 February 1977 |   | |
| 5018 Tenmu | 19 February 1977 |   | |
| 5082 Nihonsyoki | 18 February 1977 |   | |
| 5454 Kojiki | 12 March 1977 |   | |
| 5466 Makibi | 30 November 1986 |   | |
| 5541 Seimei | 22 October 1976 |   | |
| 6031 Ryokan | 26 January 1982 |   | |
| 6218 Mizushima | 12 March 1977 |   | |
| 6818 Sessyu | 11 March 1983 |   | |
| 6846 Kansazan | 22 October 1976 |   | |
| 7104 Manyousyu | 18 February 1977 |   | |
| 7105 Yousyozan | 18 February 1977 |   | |
| 7562 Kagiroino-Oka | 30 November 1986 |   | |
| 7627 Wakenokiyomaro | 18 February 1977 |   | |
| 7634 Shizutani-Kou | 14 November 1982 |   | |
| 7991 Kaguyahime | 30 October 1981 |   |
| 8133 Takanochoei | 18 February 1977 |   | |
| 8144 Hiragagennai | 14 November 1982 |   | |
| 9147 Kourakuen | 18 February 1977 |   | |
| 9153 Chikurinji | 30 October 1981 |   | |
| 9293 Kamogata | 13 December 1982 |   | |
| 9719 Yakage | 18 February 1977 |   | |
| 10006 Sessai | 22 October 1976 |   | |
| 10008 Raisanyo | 18 February 1977 |   | |
| 10009 Hirosetanso | 12 March 1977 |   | |
| 10453 Banzan | 18 February 1977 |   | |
| 11254 Konkohekisui | 18 February 1977 |   | |
| 11255 Fujiiekio | 18 February 1977 |   | |
| 11442 Seijin-Sanso | 22 October 1976 |   | |
| 11827 Wasyuzan | 14 November 1982 |   | |
| 12186 Mitukurigen | 12 March 1977 |   | |
| 12221 Ogatakoan | 14 November 1982 |   | |
| 12682 Kawada | 14 November 1982 |   | |
| 14313 Dodaira | 22 October 1976 |   | |
| 14314 Tokigawa | 18 February 1977 |   | |
| 14315 Ogawamachi | 12 March 1977 |   |
| 14316 Higashichichibu | 12 March 1977 |   | |
| 14338 Shibakoukan | 14 November 1982 |   | |
| 14795 Syoyou | 12 March 1977 |   | |
| 14820 Aizuyaichi | 14 November 1982 |   | |
| 14821 Motaeno | 14 November 1982 |   | |
| 15202 Yamada-Houkoku | 12 March 1977 |   | |
| 15671 Suzannedébarbat | 12 March 1977 |   | |
| 15672 Sato-Norio | 12 March 1977 |   | |
| 16357 Risanpei | 22 October 1976 |   | |
| 18289 Yokoyamakoichi | 22 October 1976 |   | |
| 18290 Sumiyoshi | 18 February 1977 |   | |
| 18291 Wani | 18 February 1977 |   | |
| 18322 Korokan | 14 November 1982 |   | |
| 19083 Mizuki | 18 February 1977 |   | |
| 12 March 1977 |   | ||
| 19953 Takeo | 14 November 1982 |   | |
| 19954 Shigeyoshi | 14 November 1982 |   | |
| 20962 Michizane | 12 March 1977 |   | |
| 22277 Hirado | 14 November 1982 |   | |
| 24640 Omiwa | 13 December 1982 |   |
| 26794 Yukioniimi | 18 February 1977 |   |
| 26806 Kushiike | 22 May 1982 |   |
| 14 November 1982 |   | |
| 34995 Dainihonshi | 18 February 1977 |   |
| 34996 Mitokoumon | 18 February 1977 |   |
| 12 March 1977 |   | |
| 14 November 1982 |   | |
| 14 November 1982 |   | |
| 52260 Ureshino | 22 May 1982 |   |
| 52261 Izumishikibu | 14 November 1982 |   |
| 12 March 1977 |   | |
| 12 March 1977 |   | |
| 30 November 1986 |   | |
| Co-discovery made with: K. Furukawa G. Sasaki |
|---|