The Tonight Show host, 39, pulled the stint with Diaz at the Top of the Rock in Manhattan. Tourists were told they were posing for commemorative photos for 30 Rock's website, but while they were standing for the pictures, Fallon and Diaz snuck up behind them.
The stunt aired on The Tonight Show on Wednesday, April 30. In the video, the Other Woman actress, 41, is shown giggling with Fallon as they photobomb families and groups of friends.
In one shot, the pair referenced Diaz's role in Charlie's Angels, posing with their hands in the form of finger guns, while in another they took a selfie behind the staged photo.
Watch the hilarious video above!