"Since a kid, to play in the NFL and to retire my parents has been my #1 mission," Marshall wrote. "After careful consideration but with faith, I have decided to declare for the 2021 NFL Draft."
Marshall ranks third in the SEC in receiving yards (731) and touchdowns (10). The 6-foot-3 junior from Bossier City, Louisiana, has 23 touchdown receptions over the past two seasons.
ESPN's Mel Kiper Jr. ranked Marshall as the 21st-best draft-eligible prospect and the No. 4 wide receiver on his latest Big Board.
LSU has been hit hard by opt-outs this season as fellow wideout and reigning Biletnikoff Award winner Ja'Marr Chase left school before the start of the season, as did defensive back Kary Vincent and defensive lineman Tyler Shelvin.
The Tigers are scheduled to host No. 1 Alabama on Saturday.