While performing at a benefit concert at NYC last night, Sir Elton John got very hands on ... with Channing Tatum's talented ass.

Looking like Liza Minnelli in "Cabaret," a sleeveless Tatum backed his thang up into Elton's eager hand at the Revlon Concert for the Rainforest Fund at Carnegie Hall.

Elton gives new meaning to the term tickling the ivories.