People was the first to report the shocking news, citing a source close to the couple.
"Kristen is absolutely devastated," the source said. "It was a mistake and a complete lapse in judgment."
The reported encounter occurred with her "Snow White and the Huntsman" director Rupert Sanders (pictured with Stewart at right), who is married with two children.
Us Weekly even has a shot of the couple in an embrace.
Read the rest now on TooFab: Kristen Stewart Cheated On Robert Pattinson!
More from TooFab:
- Mariah Carey Looks Super Skinny In New 'American Idol' Pic
- Guy Candy Alert: Shirtless Male Models At Miami Fashion Week
- Jennifer Lopez Celebrates 43rd Birthday With Big Boat Bash!
Photo Credit: Getty