The U Smile singer took to the stage to join Selena and her band, The Scene, during their concert at the OC Fair in Costa Mesa, CA - sending the crowd wild.
After hugging Gomez before she left the stage, JB then proceeded to sing a medley of hits, including Favourite Girl and the Justin Timberlake classic, Cry Me A River.
Following his impromptu performance, the Biebster took to Twitter, telling his followers: "Had a #greatnight and surprised the people. felt good to be on stage. nothing is changing that. #REAL (sic)."
In a separate message to Selena, the 17-year old superstar tweeted: "U did great tonight. proud of u."
Justin's appearance at the Monte Carlo star's show followed Gomez's 19th birthday celebrations, where the Baby singer is said to have treated his girl to a trip to Disneyland.
Despite reports suggesting Gomez was to spend her birthday without Justin, claiming it will be "boring", Hollywoodlife.com has reported that Biebs whisked Selena off to the California-based theme park, before taking her for a romantic meal at Japanese eatery, Geisha House.
An onlooker told the website: "They seemed really happy and totally in love. While they were eating dinner, Justin fed her a Hollywood sushi roll, which is a roll with crab, avocado and tempura cucumber."
“For dessert, they shared tempura green tea ice cream. It had three candles in it, and everyone sang ‘Happy Birthday’ to Selena, including Justin."
They added: 'Right before she blew out the candles, Justin said ‘Make a wish!’ and she smiled. After she blew out the candles, Justin leaned over and kissed her on the lips.”
It seems like Selena's birthday wasn't so boring after all...
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