Matthew Brennan Cassel (born May 17, 1982, Northridge, California) is the starting quarterback for the Kansas City Chiefs of the National Football League. He was drafted by the New England Patriots in the seventh round of the 2005 NFL Draft. He became the Patriots' starting quarterback in Week 1 of the 2008 season after then reigning NFL MVP Tom Brady suffered a season-ending knee injury. According to ESPN research, he is the only known quarterback in NFL history to start an NFL game at quarterback without ever starting at quarterback in college.

In his first season as a starter, Cassel became only the fifth NFL quarterback to have passed for more than 400 yards in consecutive games, and the first since the NFL-AFL merger with 60 rushing yards in the same game.

In February 2009, the Patriots used their franchise tag on Cassel, extending him a one-year contract worth over $14 million, the largest one-year contract for an offensive player in NFL history. Later that offseason, the Patriots made a trade which sent Cassel to the Chiefs, who signed him to a six-year, $63 million contract in July 2009.