The teenage pop star is currently touring South America and Gomez joined him during his stop in Brazil for some sightseeing.
They took a helicopter ride together in Rio on Tuesday, Oct. 4, and on Thursday, Gomez joined him onstage during his show at the Estadio do Engenhao.
The couple sang a duet of Gomez's hit track Who Says, and after the show, Bieber thanked his girlfriend for her efforts in a post on his Twitter.com page.
He wrote, "Rio!!! Crazy!! Thanks to Selena Gomez for helping me do something special for the crowd. A duet never hurt nobody. Who Says."