11:03 AM PT -- We've learned Jackson -- who goes by J.T. -- was an actor and close friend of Kevin's for more than 15 years. Jackson was in "S.W.A.T.," "Naked" and even in Kevin's movie "Think Like a Man Too." The two have posted several photos together through the years and J.T. even attended Kevin's bachelor party.

Kevin tweeted, "Mind blown ... Hurt ... at a loss for words and simply in complete disbelief at the moment. WOW."

The man who allegedly tried to coerce Kevin Hart into paying him for a sex video has been charged with extortion.

Jonathan Todd Jackson faces 2 felony counts -- attempted extortion and extortion by threatening letter.

Jackson is accused of trying to extort an undisclosed amount of money from Hart, in return for keeping the video secret ... a video showing Hart with a woman in his hotel room in Vegas apparently having sex.

The liaison went down last summer. The video is 4 minutes and 47 seconds long and highly produced. As we reported, both Kevin and the woman claim they had no idea they were being filmed.