The long and skilled 6-foot-11 forward, who averaged 17.5 points and 8.9 rebounds per game, tweeted the news on Tuesday.
According to several NBA executives, Portis is considered a lock first-rounder and could be selected late in the lottery. Portis was No. 16 on ESPN NBA Insider Chad Ford's most recent big board.
Arkansas, which lost in the round of 32 to North Carolina in the NCAA tournament, will likely take a significant hit next season without Portis, who led the team in scoring and rebounding. Athletic Razorbacks wing Michael Qualls still hasn't made a decision whether he will join Portis in declaring or whether he will return to Fayetteville.
Arkansas was 27-9 last season and 13-5 in SEC play.