Harvey Weinstein is boarding a private jet Tuesday night, bound for a rehab center in Europe for sex addiction ... sources connected with the former mogul tell TMZ.

We're told Harvey has decided to take the advice of the people around him and leave immediately. We're told he will enter a live-in facility and will deal both with sex and other behavioral issues.

Our sources say Weinstein is surprisingly calm, considering the PR nightmare engulfing him. As one source put it, "He has his moments where there are bursts, but for the most part he's pretty calm."