"My daughter just had a daughter. It’s amazing to be by your side through this journey. She’s SO beautiful already. Can’t wait to watch her grow," ther former US Olympian wrote in a caption.
"Throwback to my baby, as a baby @kyliejenner," she continued alongside a baby photo of Kylie from 1997.
"Love you ❤️," the 20-year-old Kylie Cosmetics founder replied.
Meanwhile, older sisters Kim Kardashian West and Khloé Kardashian, 33, have both congratulated Kylie on the arrival of her new baby with boyfriend Travis Scott.
With Kardashian West, 37, taking to her website to write a heartfelt letter which read: "Mummy Kylie!!! You did it! I’m so incredibly proud of the woman you have grown up to be and I know that you are going to be the best mum to your new baby girl!”
“Our girls are two weeks apart and I can’t wait for them to be besties!” she added. “I am so proud that you do what you want to do and live your life the way you want, you will pass down all of those amazing qualities to your baby girl. You were made to be a mum! I love you!”
While Khloé dedicated an Instagram post to her younger sister. "Congratulations My Sweet Kylie! What a magical ride it's been!" she wrote alongside a rare picture of the two sisters while they were both donning baby bellies.
"I'll miss you bumping along with me. Never in a million years did I think we would do this together but it's been that much more special because of it. God is so great!! He had His plan all along! I love you little mama 💕 love big mama," she concluded.
So much love from the Kardashian Klan!