Otto Hilgard Tittmann (August 20, 1850 – August 21,^[1] 1938) was an American geodesist, geographer, and astronomer of German descent.

Tittmann was born in 1850, in Belleville, Illinois^[2] to revolutionary parents fleeing the aftermath of the 1848 revolutions.^[3] He attended school in St. Louis, and in 1867 joined the United States Coast Survey.^[2] In 1874 he was assistant astronomer in Japan to view the Transit of Venus and from 1889 until 1893 he was in charge of weights and measures. In 1888 he co-founded the National Geographic Society. In 1899, Tittmann served as president of the Philosophical Society of Washington.^[4] And, from 1895 until 1900 he was assistant in charge of the United States Coast and Geodetic Survey. He was elected to the American Philosophical Society in 1906.^[5] From 1900 until 1915 he was Superintendent of the Survey, and from 1915 until 1919 he was president of the National Geographic Society. He died in Leesburg, Virginia, in 1938.