Baron Karl-Robert von Osten-Sacken (1828, St. Petersburg –1906, Heidelberg) was a Russian diplomat and entomologist. He served as the Russian consul general in New York during the American Civil War, living in the United States from 1856 to 1877.
He developed an early interest in entomology specialising in Diptera and especially the Tipulidae. In 1862 Osten-Sacken published, with assistance from Hermann Loew, “Catalogue of the described Diptera of North America” in Smithsonian Miscellaneous Collections Vol. 3. A later edition of this work appeared in 1878, as Smithsonian Miscellaneous Collections no. 270. He published many other papers. His work on the Tipulidae included a classification of the family. He also studied insect galls and worked on the Tabanidae. Osten-Sacken corresponded with Hermann Loew, supplying him with specimens, and translated and published Loew's work in the 'Monographs of the Diptera of North America', (1862-1873), Smithsonian Miscellaneous Collections, Nos 6, 171, 219 and 256.
Asteroid 335 Roberta is named in his honour.