Justin Bieber Surprises Selena Gomez Onstage & Sings Justin Timberlake's "Cry Me a River"

During Selena Gomez's concert at Pacific Amphitheatre in Costa Mesa, Calif., on Sunday, boyfriend Justin Bieber made a surprise appearance and the fans went wild!

OK! NEWS: JUSTIN BIEBER TWEETS SELENA GOMEZ HAPPY BIRTHDAY & PROMISES IT WILL BE "SPECIAL"

Another reason Justin would make for a very good boyfriend!

Justin, wearing a Yellow A|X Armani Exchange V-Neck Pima T, surprised an delighted Selena onstage, Celebuzz reports. He gave her a big hug before she gave him the floor to sing "Favorite Girl" and Justin Timberlake's "Cry Me a River."

The 17-year-old singer made sure to Tweet Selena after the show and congratulate her, writing, "U did great tonight. proud of u."

So sweet!

He also Tweeted, "Had a #greatnight and surprised the people. felt good to be on stage. nothing is changing that. #REAL (sic)."

OK! GALLERY: HAPPY 19TH BIRTHDAY, SELENA GOMEZ — FROM DISNEY PRINCESS TO JUSTIN BIEBER'S QUEEN

The joint performance looked like a good end to Selena's 19th-birthday weekend celebration.

In other Bieber news, the hashtag #sorryjustin has become a Twitter trend today after a fan apologized for creating fake DM's from the star that made him seem rude, Bieber-Fever.net reports.

Watch a fan-made video YouTube clip of Justin's surprise appearance, below!