Talk about shocker!

When Mario Lopez published his memoir "Just Between Us" in September, he admitted that he had once had a one-night stand with a "mega pop star" in Las Vegas. But he left fans wondering who the mystery woman could be.

Well, a source has now revealed to Us Weekly that Lopez's sin city liaison was with the one and only Britney Spears.

Lopez appeared on the "Ellen Show" on Oct. 2 to promote the book and played coy when asked about the one-night stand.

"I'm trying not to give too much away without giving too much away," Lopez told Ellen DeGeneres. "It was maybe six or seven years ago... eight years ago?"

When DeGeneres asked if Lopez was still on good terms with the unnamed pop star, the 41-year-old television host said the two remained "friendly."

Lopez also added that, out of respect for his wife Courtney Mazza, with whom he shares two children, he would not reveal the name of the woman from the steamy night in Vegas.

But it looks like that secret may now be out.

It is unclear if Lopez and Spears were single at the time of the tryst, which seemingly occurred around 2006.

The "...Baby One More Time" hitmaker filed for divorce from husband Kevin Federline in Nov. 2006, just two months after giving birth to their second son.     

As for Lopez, he dated his "Dancing With the Stars" partner Karina Smirnoff on-and-off from 2006-2008.

Lopez and Spears certainly appear to have remained on good terms. They have done several interviews together since that time and the "X-Factor" host even joined Spears on stage in 2013 for the opening night of her Las Vegas residency show "Britney: Piece of Me."