The brunette beauty saw in the milestone at the Bagatelle restaurant in West Hollywood without her on/off boyfriend Justin Bieber, but she was joined by her mother Mandy Cornett and stepdad Brian Teefey, who purchased some alcoholic beverages for her group of friends, the New York Post newspaper reports.
Selena - who recently said she intended to have her first legal alcoholic drink at the eatery - was spotted popping bottles of champagne to celebrate and at one point she started dancing on the bar.
Sexy show
The pop star posted a picture on Instagram of her surrounded by her group of friends and a large cake, which featured a big 21 written in icing, and wrote: "Thank you to bagatellela for an amazing bday!!!! It's a birthday WEEK! #werk #becauseIcan (sic)"
The singer-and-actress was also accompanied at Bagatelle by her Spring Breakers co-star Ashley Benson, and the pair and Selena's friends went on to watch a sexy show and party at Beacher's Madhouse.
The group had VIP seats to take in TV personality and former stripper Busty Heart, who has a talent for crushing beer cans by banging her breasts on them.
Selena posted the following photo on Instagram: