Adam Silver just announced the historic punishment on Wednesday ... explaining league investigators discovered that in March, Porter took part in a gambling scheme that ultimately called into question the integrity of the NBA's competition.

They claim the 24-year-old gave confidential information about his health to at least one known bettor. They say another bettor Porter knew then placed an $80K wager that Porter "would underperform" in the Raptors' March 20 tilt with the Sacramento Kings.

The league said the bettor stood to make $1.1 MILLION if the wager hit.

NBA officials say Porter pulled himself from the contest early -- due to what he claimed was an illness. But, the league said that because of the "unusual betting activity" ... the bet was "frozen" and "not paid out."

Additionally, Silver revealed NBA investigators found that while Porter was playing for both the Raptors and their G-League affiliate from January through March, he made "at least 13 bets on NBA games."

Silver said Porter used an associate's online betting account to make the wagers, and scored over $20,000 in profits.

"None of the bets involved any game in which Porter played," the league said in a statement. "Three of the bets were multi-game parlay bets that included one Raptors game, in which Porter bet that the Raptors would lose. All three bets lost."

Silver and the NBA said they're in contact with the feds -- and have promised to continue to investigate Porter ... and turn over any future findings.

"There is nothing more important than protecting the integrity of NBA competition for our fans, our teams and everyone associated with our sport," Silver said, "which is why Jontay Porter’s blatant violations of our gaming rules are being met with the most severe punishment."