William Clay Matthews III (born May 14, 1986) is an American football outside linebacker for the Los Angeles Rams of the National Football League (NFL). After attending Agoura High School in Agoura Hills, California, Matthews was a walk-on student athlete at the University of Southern California for the USC Trojans football team under head coach Pete Carroll. At USC, Matthews was a standout special-teams player, winning three consecutive Special Teams Player of the Year awards from 2006 to 2008. He also played reserve outside linebacker during those years before moving into a starting role his senior season. During his college career, he was a part of three Pac-10 Championship teams.

Matthews was considered a top prospect for the 2009 NFL Draft. He was ultimately selected by the Packers in the first round of the draft (26th overall) after the team traded up to make the selection. In his rookie year, Matthews recorded 10 sacks while playing outside linebacker. He topped that total in 2010 with 13.5 sacks, helping the Packers to their Super Bowl XLV victory against the Pittsburgh Steelers. Matthews continued his role as a leading pass rusher, recording at least six sacks in the first nine seasons he played. He also has showed his athleticism and abilities by playing both inside and outside linebacker during the 2014 and 2015 seasons.

A member of the Matthews family of football players, he is the son of former NFL linebacker Clay Matthews Jr. and nephew of Pro Football Hall of Fame offensive lineman Bruce Matthews.