Charlie Sheen has an upcoming autobiography and a Netflix documentary about his life … and while his ups and downs will be well-documented, TMZ has learned something he’s never talked about … he’s been one of the most charitable celebs in Hollywood.

The actor’s going through some lean years financially, but sources with direct knowledge tell us when Charlie was rolling in money — in the “Two and a Half Men” days — he donated between $26 million and $30 million to various charities.

We’re told Charlie’s massive donations were spread around a few noble causes … including Autism Speaks, cancer research, and projects aimed at helping the homeless.

Thing is … at the time, no one knew Charlie was behind the donations. Our sources say all of the money was donated anonymously, upon Charlie’s insistence.

We’re told Charlie didn’t donate to get recognition or credit … instead, we’re told he donated because he felt like it, and he made it clear to his team no one was to utter a word.

Yep, more evidence Charlie’s cut from a different cloth!