William Peter Filson (born September 28, 1958) is a retired Major League Baseball pitcher.

He played during seven seasons at the major league level for the Minnesota Twins, Chicago White Sox, New York Yankees, and Kansas City Royals.

Filson can be seen delivering a single pitch as a Royal in the Ken Burns documentary Baseball, at the beginning of the section entitled "Extra Innings."

He was dealt along with Larry Milbourne and John Pacella from the Yankees to the Twins for Butch Wynegar and Roger Erickson on May 12, 1982.

Filson is currently a pitching instructor at AFC Baseball in Bala Cynwyd, Pennsylvania, where he provides instruction to players at Harriton High School, Radnor High School, the Chester County Crawdads, and Wayne Wolverines.