Carl Robert Osten-Sacken or Carl-Robert Romanovich, Baron von der Osten-Sacken (21 August 1828, St. Petersburg – 20 May 1906, Heidelberg) was a Russian diplomat and entomologist. He served as the Russian consul general in New York City during the American Civil War, living in the United States from 1856 to 1877. He worked on the taxonomy of flies in general and particularly of the family Tipulidae (crane flies).
Robert Osten-Sacken took an interest in insects at the age of eleven through the influence of Joseph N. Schatiloff, a Russian coleopterist. 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, 256. He proposed the term chaetotaxy.Asteroid 335 Roberta is named in his honour. A subspecies of North American snake, Thamnophis sauritus sackeni, was named in his honour by Robert Kennicott in 1859.