Justin Bieber and Taylor Swift have been friends for a while, he used to open for her.  But Tuesday night, the Biebs actually joined Taylor onstage during her concert at L.A.'s Staples Center, finally making good on a comment he made via Twitter earlier this summer.

Back in July, Justin tweeted to Taylor, "I heard you sang 'Baby' at your concert in Toronto. Thank u for the love...might have to come by and stop in on a show."  And Tuesday night, he did, popping up onstage and singing "Baby" as a duet with Taylor.  Afterwards, she 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."  Justin added, "@taylorswift13 we should do that more often. :) great show! #muchlove."

Taylor's shows in L.A. continue Wednesday night, Saturday and Sunday.

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