Justin Bieber popped up at Selena Gomez’s concert at the in Costa Mesa, Calif over the weekend and sang to her on stage, while fans went wild.

Although Gomez is still coy about the extent of their relationship, Justin has gone out of his way to show his affection for the 19-year-old former Disney star.

He appeared on stage and gave her a huge hug, before launching into his songs. It looked entirely planned.

Justin, 17, sang a medley of hits, including “Favorite Girl” and a cover of Justin Timberlake‘s “Cry Me A River.”

Selena got into the groove and was dancing next to him.

“u did great tonight. proud of u,” Justin Tweeted after the show.

Two days earlier (July 22), Justin and Selena spent her birthday together.

“Happy Birthday @SelenaGomez gonna be a special day.” the Biebs Tweeted.

Singer and friend Camryn hung out with them at a donut shop and later tweeted a pic: “just hanging out with @selenagomez & @justinbieber at the donut shop. #NBD.”

Justin also posted a shot giving Selena a kiss: “Happy bday baby.” he wrote. What a charmer!

The couple dined out at Japanese restaurant Geisha House in Hollywood. Check out the Biebs on stage below.