Guests such as Dancing with the Stars dancer Julianne Hough and actresses Ashley Benson and Nina Nobrev danced the night away at the birthday bash.
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Justin made a surprise appearance at the party, prompting rumours of a reconciliation with the former Disney star.
The Never Say Never singer arrived at The Revolve Beach House in Malibu in his leopard print Audi sports car with several friends, and was later spotted with fellow pop star Jaden Smith, who was already at the party.
It is reported that Justin then left his sports car at the beach house and jumped into a car with the birthday girl around 2am.
Although the on/off couple have been spotted together in recent weeks, it was rumoured that Justin's invitation was not sent, following reports that he had made no effort to give Selena a present to celebrate the big milestone.
Regardless of her ex-boyfriend's generosity, earlier in the day Selena thanked fans who had given her the best birthday present by helping her record reach the top of the charts.
"I just want to say thank you all for an amazing birthday week. The success of this album has been the greatest gift!" tweeted the Come And Get It singer.
Earlier in the week, Selena appeared on Good Morning America and opened up about her turbulent relationship with Justin.
"He and I have such a history together and I'll always care about him and protect him if anybody tries to be mean," she said.
Selena and Justin broke up in November 2012 after two years together. The teen stars met in 2010 and confirmed their relationship in February 2011.