The All-American defensive back is ranked fifth overall on Mel Kiper's latest Big Board of NFL draft prospects and is the highest-ranked cornerback. Williams is widely considered to be a first-round pick in the upcoming draft. He is LSU's active career leader in interceptions (eight) and pass breakups (20).
The 6-foot-2, 182-pound Williams was a third-team Associated Press All-American in his redshirt-freshman season of 2017 and a preseason and midseason All-American this season. He's a finalist for the Thorpe Award, which goes to the nation's top defensive back.
Williams announced his intentions on his Twitter account on Sunday afternoon.
The timing of Williams' announcement came to the surprise of LSU head coach Ed Orgeron. He received the news while he was on a teleconference for the PlayStation Fiesta Bowl, where the Tigers will play No. 8 UCF Knights on Jan. 1.
"If that's what he wants to do, we wish him and his family the best," Orgeron said. "I didn't know it was posted yet. We did have a meeting. We were going to have a final meeting, but if that's what him and his family want.
"I haven't met with all my juniors yet, so I was going to announce it at the end when I had a final meeting with everybody, but we wish him the best. He's a tremendous young man, him and his family."
Orgeron also had news on another LSU cornerback, as Kristian Fulton will miss the team's bowl game after having ankle surgery. Fulton has not played since a Nov. 10 win over Arkansas.
ESPN's Andrea Adelson contributed to this report.