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.

Expulsion is the most serious form of disciplinary action that can be taken against a Member of Congress. The United States Constitution provides that "Each House [of Congress] may determine the Rules of its proceedings, punish its members for disorderly behavior, and, with the concurrence of two-thirds, expel a member." The process for expulsion from the House of Representatives differs somewhat from the process for expulsion from the Senate.