She wrote: "Momma, Brian, and me are all so grateful for all your well wishes. My lil sissy Gracie Elliot Teefey was born on June 12th. Luv you all XO."
Mandy divorced Selena's father, Ricardo Gomez, when the Love You Like A Love Song singer was five-years-old, before marrying Brian Teefey in 2006.
Demi Lovato was quick to "congratulate" her life-long friend on becoming a "big sister" when she made the comment on her own Twitter account today.
She tweeted: "@selenagomez IS A BIG SISTER!!!!!! Everyone congratulate her and her family!!!!!! #CONGRATSSELENA!!!!"
Speaking before Gracie's birth, Selena admitted she would "spoil the baby" and wanted to be "more like a cool aunt" in the future.
She previously said: "I want to be the best sister in the world. I'm really going to spoil the baby! I can't wait ... There's going to be a big age difference, so I think I'm going to be more like a cool aunt."