William Sebastian Jacobsen (January 15, 1887 – April 10, 1955) was a Democratic U.S. Representative from Iowa's 2nd congressional district who served three terms from 1937 to 1943.

Born in Clinton, Iowa, Jacobsen attended the public schools and the Normal College of American Gymnastics Union, in Indianapolis, Indiana.

In June 1936, Jacobsen's father was a candidate for a fourth term in Congress, and had just won the Democratic primary when he became ill and died.

In the last four years of his tenure, Democrats from Iowa in the House delegation outnumbered Republicans by a two-to-one margin.

The 1938 election would reverse that trend, leaving William Jacobsen and Vincent Harrington as the only Iowa Democrats in the House.