Otto Wilhelm Sonder (18 June 1812, Bad Oldesloe – 21 November 1881) was a German botanist and pharmacist who was a native of Holstein. From 1841 to 1878 he was the proprietor of a pharmacy in Hamburg. In 1846 he received an honorary doctorate from the University of Königsberg.
Sonder kept a very extensive botanical collection, containing hundreds of thousands of specimens. After it grew too large for him to manage on his own, he sold it to his friend Ferdinand von Mueller, who sold parts of it to the Swedish Museum of Natural History and to Jean Michel Gandoger, with the bulk of the collection, about 250-300,000 specimens, going to the Melbourne Royal Botanic Gardens.
He was co-author, with William Henry Harvey (1811-1866), of Flora Capensis (7 vol. in 11, 1859–1933), and was author of an 1851 botanical treatise called Flora Hamburgensis.
The plant genus Ottosonderia of the family Aizoaceae is named in his honor.