The Canadian pop star arrived with the Wizards Of Waverly Place star and they were seen hand-in-hand backstage together before Justin closed the show at KIIS-FM's Jingle Ball, E! reported.
Their appearance at the two-night music extravaganza suggests that recent talk of them splitting up is wide of the mark.
According to E!, Justin then flirted with his fans when he took to the stage at the Nokia Theatre, asking them: "I see a lot of girls out there and I just want to know how many of you are single?"
He also lifted up his t-shirt to flash the top of his boxers during his big hit Baby.
The Die Young singer wore a raunchy black and gold leotard and boots and rocked out with an electric guitar during her set.
Also appearing were Gangnam Style star Psy, fun and Flo Rida.