The Come & Get It hitmaker spent her Saturday night (20Jul13) at a star-studded 1920s-themed party hosted by fellow birthday girl Julianne Hough, who turned 25 at the weekend, before heading to Hollywood hotspot the Roosevelt Hotel with Collins.
She enjoyed her own small gathering on Sunday (21Jul13), the eve of her birthday, when she was joined by more than a dozen friends, who all donned party hats and treated her to tacos from American fast food restaurant Jack in the Box.
Gomez shared a photo of the festivities on Instagram.com, alongside the message, "Guess what... I'm 21. Um, turnupNow... My cake was... Jack n the box (sic) tacos... Yes."
She also marked her big day on Monday (22Jul13) by posting a special message and video, featuring her song Birthday, on YouTube.com.
Directing fans to the promo, she wrote, "I wanted to celebrate my 21st Birthday with all of you, so we made this video for us all to celebrate to. Thank You for all the love and support you have shown and continue to show me. I love you all so much!!
"Thanks to all my amazing friends... Thanks to all my fans for being so amazing and making me want to make this video for you."
Meanwhile, Taylor Swift took to her Twitter.com page on Monday to send her best wishes to her pal, writing, "Happy 21st birthday to my friend, and the closest thing I've ever had to a sister, @selenagomez!"
Gomez's big day fell on the same day Britain's Prince William and Catherine, Duchess of Cambridge welcomed their first child, a son.