In honor of Khloé, big sis Kourtney Kardashian called her the "definition of #bodygoals," and reminisced about some of the times she's looked "smoking hot."
"Happy birthday, Khloé! I'm so proud to call you my sister! You are one of the strongest, most determined people I know, and I love going through life together," she wrote. "Thanks for being my best friend and the best Auntie Koko to my babies!"
Khloé's little sister Kendall Jenner also made an app announcement, but hers included a joke.
VIDEO: Get That Body: Khloé Kardashian's Booty
"Happy birthday, Khloé! I hope you have the best 21st birthday yet, lol," Jenner wrote. "I love you!"
While Khloé's youngest sister Kylie Jenner didn't post a birthday wish on her app for Khloé (at least not yet!), she did make sure to come to Khloé's adorable surprise party over the weekend, and that's what counts.
Happy birthday, Khloé!