Justin Bieber and Selena Gomez Duet in Rio [VIDEO]

Justin Bieber surprised thousands of screaming fans in Rio de Janeiro on Thursday night when he brought out girlfriend Selena Gomez on stage for a special duet.

Bieber, 17, disappeared for a brief moment before returning to duet with Gomez, 19, on her hit single, Who Says.

The Canadian singer is currently on the South American leg of his My World tour, which kicked off in Rio de Janeiro on Oct. 5.

This is not the first time the two have performed on stage together.

In July, Bieber surprised Gomez fans by joining her onstage in Costa Mesa, Calif. for a performance of Justin Timberlake's Cry Me A River.

Naturally, Bieber took to Twitter to comment on the experience.

Had a #greatnight and surprised the people. felt good to be on stage. nothing is changing that. #REAL . . . @selenagomez u did great tonight. proud of u, he wrote.

Since confirming their relationship in March, Bieber and Gomez have not been shy about being seen in public.

Earlier this week, they were photographed taking a romantic helicopter ride over Rio de Janeiro. And in September, Bieber took Gomez to the Staples Center in Los Angeles for a private screening of Titanic.

Following the surprise duet on Thursday, Bieber once again used Twitter to thank his fans and girlfriend.

RIO!!! CRAZY!! thanks to @selenagomez for helping me do something special for the crowd. a duet never hurt nobody. WHO SAYS, he wrote.

The singer is scheduled to hit Sao Paulo, Buenos Aires and other cities before wrapping the tour in Caracas, Venezuela on Oct. 19.

Scroll down to watch a video of Bieber and Gomez's duet in Rio.