Kendall Jenner took to her app, writing, "I hope you have the best birthday, Ky! I can't believe my little sister is officially not a teenager. We've had so many amazing memories over all the years. I love you!"
Kim Kardashian West echoed Kendall's sweet sentiments on her own app, telling Kylie she's "so proud" of all she's accomplished.
"I'm so inspired by how hard you work every day," she said. "I hope you have the best birthday! Love you!"
Kim continued the love on Instagram, sharing a sexy photo of her "baby sis" hanging out poolside.
"You are always so determined & so set on what you want in life & starting at such a young age you always went for it," Kim wrote. "That's so inspiring to watch. I love you Ky."
"You are one of a kind, and I'm grateful every day that God gave me you," she continued. "I love being your mommy! There is so much ahead for you, my angel, and I can't wait to see what your 20s has in store!! I love you more than you will ever know! Mommy xo."
The birthday girl also took to her own app, revealing her favorite moments from the last year that have made her life so great: Creating The Kylie Shop, attending the Met Ball with Donatella Versace and modeling for a calendar shoot were just a few that made the list.
Hear more highlights from Kylie's fierce and fabulous year in the video below!