Fans at Selena Gomez's Orange County concert on July 24 were treated to a scream-inducing surprise when her beau Justin Bieber took the stage to perform a song of his own -- apparently a surprise for Selena, too.

The Biebs performed a mash-up of his song 'Favorite Girl' with another Justin's tune -- Timberlake, of course. The Biebs put his soul into J-Tim's 2002 hit 'Cry Me a River' over a sea of shrills.

Bieber was incredibly easy to spot on the stage, being that he was sporting a bright yellow t-shirt and carrying a sparkly microphone, and Selena fans had no problem recognizing her main man. He entered the room as a backup guitarist began playing the Timberlake track on an acoustic guitar, but stopped center-stage to hug his girlfriend, who was obviously elated.

Like a good heartthrob, Biebs made sure the girls were screaming to their full potential before he really dove into the song. The 'Baby' hitmaker truly put his heart into his performance, and his rendition of the cover track came out flawlessly and effortlessly. Before he made his exit, Bieber shouted, "Give it up for Selena Gomez!" into his glitter-mic, which lent to even more screaming.

We do feel the need to point out that while we know Selena is Justin's 'Favorite Girl,' 'Cry Me a River' is a break-up song ... And a very spiteful one at that. We're sure Justin's Beliebers will love to create theories based on that alone!

So much for that month-long break from the mic, huh Biebs?

Watch Justin Bieber Perform 'Favorite Girl' and 'Cry Me a River'