"You think you've heard LOUD screaming in your life.. Then @justinbieber comes out and does a surprise song during your show. Woah. Unreal," she Tweeted after the concert.
"@taylorswift13 we should do that more often," Justin replied.
":) great show! #muchlove."
Taylor's known for bringing along someone a bit special to her shows; she did a duet of Hot 'n' Cold last year with Katy Perry, and brought on John Mayer the year before.
Justin slowly walked out from the back of the stage at Taylor's gig in LA's Staples Center, before rushing up the staircase she was standing on to sing next to her for a second until heading back down to sing closer to the crowd.
Check out the video...
Click next to for an even closer look at the duet, and to re-live Taylor's duet with Katy Perry from last year!