The get-up struck one customer as so funny that he started joshing with the Darth Robber after he strode into the Chase bank in Setauket.
"The customer thought it might have been a joke, and not a serious attempt at a robbery," said Suffolk County police Detective Sgt. William Lamb.
But Darth wasn't kidding - and he wasn't going to be stopped by a non-Jedi Knight.
He won a "shoving match" against the incredulous customer before using his piece to order him to the floor, Lamb said.