The former Disney star marked the milestone birthday on Monday, 22 July, and wasted no time in taking to Twitter to thank fans for their well-wishes.
Spring Breakers star Selena released her 'Birthday' music video this morning, adding the caption: "I wanted to celebrate my 21st birthday with all of you, so we made this video for us all to celebrate to."
She added: "Thank you for all the love and support you have shown and continue to show me. I love you all so much!"
Check out the video below – we warn you, the chorus will get stuck in your head for the rest of the day!
The video shows Selena and her pals dancing around, having fun, chucking lots of colourful confetti and swinging lots of long, shiny hair at the camera.
Selena's dressed in a crop top and black shorts, with a checked shirt tied around her waist and, at one stage, finds herself a princess tiara. Well, it's her birthday!
'Birthday' is taken from Selena's new album Stars Dance, which also features the hit single 'Come & Get It'. She'll embark on a world tour in support of the record later this year.
Selena was snapped celebrating her big day with friends, including actress Lily Collins, in Los Angeles last night, donning a pair of devil horns for the occasion.
We wonder if Justin Bieber got her a gift…