Gen-ichi Koidzumi (小泉 源一, Koizumi Gen'ichi, 1 November 1883 – 21 December 1953) was a Japanese botanist, author of several papers and monographs on phytogeography including work on roses and Amygdaloideae (Rosaceae), maples (Aceraceae), mulberries (the genus Morus), and many other plants.

Gen-ichi Koidzumi was born in Yonezawa in Yamagata Prefecture in 1883.

[2] After graduating from the Sapporo Agricultural College, he studied biology at Tokyo Imperial University from 1905, continuing his studies there under Matsumura Jinzō, and receiving his doctorate in 1916.

[2] In 1919, he was appointed assistant professor at Kyoto Imperial University, where he remained (other than for a tour of the herbaria of Europe and the United States from 1925 to 1927) until his retirement in 1943; he was promoted to full professor in 1936.

[2] In 1932, he founded the Societas Phytogeographica and the journal Acta Phytotaxonomica et Geobotanica.