Justin Bieber is a man of his word! After Tweeting to Taylor Swift that he'd join her on stage, the 17-year-old teen sensation came through.

During Taylor's LA concert on Tuesday, Biebs surprised the audience by joining the country cutie on stage for a performance of "Baby."

Following her concert, Swift Tweeted:

    "You think you've heard LOUD screaming in your life.. Then @justinbieber comes out and does a surprise song during your show. Woah. Unreal."

Taylor's BFF also happens to be Selena Gomez, so that may have helped, too!