William King Sebastian was an American politician, judge, and lawyer from Helena, Arkansas. He represented Arkansas as a United States Senator, from 1848 to 1861. Sebastian withdrew from the Senate at the start of the Civil War and was later formally expelled by the Senate. He took no active part in the Confederate government and was reinstated by a posthumous resolution in 1877.

The Sebastian Indian Reservation (1853-1864), more commonly known as the Tejon Indian Reservation, was formerly at the southwestern corner of the San Joaquin Valley in the Tehachapi Mountains, in southern central California.