A speedy All-American who transferred to Alabama from Ohio State, Williams led the SEC in receiving yards (1.572) and touchdowns (15) this season.
ESPN's Mel Kiper Jr. lists Williams as the No. 1 draft-eligible receiver and sixth-best prospect regardless of position.
Complicating Williams' draft standing is an injury he suffered against Georgia in the College Football Playoff National Championship game on Monday.
Sources told ESPN's Adam Schefter that Williams tore the ACL in his left knee.
Doctors believe that once surgery is completed within the next week, Williams will retain his sub-4.3 speed, and they expect a full recovery, the sources said.
Williams wasn't the only Alabama player to declare for the draft on Thursday.
Offensive tackle and potential top-five pick Evan Neal also announced that he would be leaving school early.
Defensive tackle Phidarian Mathis, a top-10 prospect at his position according to Kiper, declared for the draft on Wednesday.
Alabama is still awaiting announcements from multiple draft-eligible players, including wide receiver John Metchie III.
Metchie led Alabama in receptions (96) and is Kiper's No. 8-rated receiver.