"I'm making it official," Simmons said late Sunday night.
The 6-foot-10 Australian averaged 19.2 points, 11.8 rebounds and 4.8 assists per game this past season for the Tigers, who finished a disappointing 19-14 overall and 11-7 in the SEC, failing to make the NCAA tournament.
LSU declined an invitation to play in the NIT.
Simmons entered the season as the top freshman in the country and was the only player to average more than 19 points, 11 rebounds and four assists.
According to numerous NBA executives, Simmons is expected to be taken either first or second in the June 23 NBA draft.