Williams led the all-quarterback final four with 2,031 points once the votes were counted.

Texas Christian University (TCU) Horned Frogs signal-caller Max Duggan was second on 1,420.

The Ohio State Buckeyes' CJ Stroud was a distant third and Stetson Bennett IV from the Georgia Bulldogs fourth.

Williams leads all of top-level college football in passing touchdowns this season, with 37, and is fourth in total passing yards with 4,075.

This is his first season in USC colours after he followed head coach Lincoln Riley from the Oklahoma Sooners to Los Angeles.

Williams is the third USC quarterback to win the Heisman and the school's record eighth winner - although the official list says seven.

Reggie Bush won it in 2005, but that is considered vacated because Bush broke National Collegiate Athletic Association rules by accepting gifts.

"I've watched it and to now be a part of this fraternity is a blessing," Williams said.

"It's everything I've dreamed of."

Williams, a college sophomore, is the only one of finalists not due to play in the College Football Playoff.

Defending champions Georgia face Ohio State in one semi-final and TCU are up against the Michigan Wolverines in the other.