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