William K. Brewster (born November 8, 1941) is a former American politician who represented Oklahoma in the United States House of Representatives. A member of the Democratic Party, he served as the U.S. Representative for Oklahoma’s 3rd Congressional District from January 3, 1991, to January 3, 1997. During this period, he completed three terms in Congress.

Brewster’s tenure in the House placed him among the historical members of Congress from Oklahoma in the late twentieth century. Serving as a Democratic representative for the 3rd District, he participated in the legislative work of the U.S. House during a time of shifting political dynamics both in Oklahoma and nationally. He left office at the conclusion of his third term in 1997 and has since been regarded as a former member of Congress.