Milas Kunis is letting out a big secret — her naked butt in Friends with Benefits isn’t hers. But, RumorFix is happy to report — the boob is!

Mila talked to Ryan Seacrest on KIIS-FM Tuesday morning and said, “I showed side boob and figured ‘I can’t give away everything all at once.”

So they looked for a butt double for the romantic comedy.

“These young ladies came in,” Mila said. “and they were in a casting room, the director and the casting director and my makeup artists and these lovely women had to show us their derrière and we chose the one that resembled my body the most without looking like it is fake.”

By the way, Mila’s going on the record as saying her co-star Justin Timberlake has a nice backside!