Guess what?

Former United States Air Force airman, Boston mob kingpin and convicted murderer Whitey Bulger is a lefty.

That’s according to documents from the Florida prison where he’s being incarcerated.

They know he’s a southpaw because one of their guards caught him using it to pleasure himself with the lights on at 3 a.m. last June — as first reported by the Boston Globe.

“I got you!” the guard at the federal penitentiary in Sumterville, Florida exclaimed when he saw Bulger (no pun intended) unabashedly flogging his sinister log.

The inmate was subsequently written up for the demonstration. The prison prohibits its inmates from indulging in any sexual activity.

More from the New York Daily News:

His personal manhandling earned him 30 days in solitary confinement and the loss of commissary and email privileges for the same time period, The Globe said.

Bulger, who was convicted in 2013 of 11 murders while overseeing a widespread criminal ring in Boston during the 1970s and ‘90s, said he’d been “set up” by the correction officer.

“I’ve never had any charges like that in my whole life. I’m 85 years old. My sex life is over,” Bulger said at a disciplinary hearing, according to The Globe.

The guard saw him applying a special medicated powder to his genitals in an effort to treat a yeast infection, Bulger said.

“I ended up with a condition and I’m embarrassed to go to medical because they have female nurses over there,” Bulger said.

As for why he keeps his lights on all the time, Bulger blamed his horrible nightmares — a byproduct an LSD experiment he participated in while locked up in an Atlanta federal penitentiary for a bank robbery.