Otto Christian Leonor Gelert (9 November 1862 in Nybøl on Sundeved peninsula – 20 March 1899 in Copenhagen) was a Danish pharmacist and botanist, who specialized in plant floristics and systematics. He was a brother of sculptor Johannes Gelert.

In 1883 he received his degree in pharmacy and subsequently worked as a druggist in the communities of Ribe and Horsens. From 1894 he worked as chemist at a sugar refinery in Tangermünde, Germany.^[1] During the latter part of his career he worked at the botanical museum in Copenhagen. In 1897 he took a scientific excursion to the Canary Islands. In 1899 he died of tuberculosis in Copenhagen, aged 36.^[2][3]

Plants with the specific epithet of gelertii commemorate his name, and example being Taraxacum gelertii.^[4]

• Danmarks og Slesvigs Rubi, (with Kristian Friderichsen), 1887 – Danish and Schleswig Rubus.
• Studier over Slægten Batrachium, 1896 – Studies of the genus Batrachium.
• "Notes on Arctic Plants", in: Botanisk Tidsskrift, vol. 21, 1898.^[2]
• "Flora arctica, containing descriptions of the flowering plants and ferns found in the Arctic regions, with their distribution in these countries", 1902 (edited by Carl Hansen Ostenfeld).