The 68-year-old television personality broke her silence on Jenner's baby news in an Instagram post Monday after Kylie welcomed a daughter with Travis Scott.
"My daughter just had a daughter," Caitlyn captioned a throwback photo of Kylie as a baby. "It's amazing to be by your side through this journey."
"She's SO beautiful already," she said of Kylie's daughter. "Can't wait to watch her grow. Throwback to my baby, as a baby @kyliejenner."
Caitlyn shares Kylie and 22-year-old daughter Kendall Jenner with her ex-wife Kris Jenner. Kris also celebrated Kylie's baby news after the 20-year-old reality star and cosmetics mogul announced her daughter's birth Sunday.
"God is SO good!! I am so excited and thrilled to welcome another precious baby girl to our beautiful, ever growing family. We are blessed beyond belief!" Kris tweeted.
Caitlyn came out as transgender in April 2015 after splitting from Kris in 2014 after nearly 23 years of marriage. The former couple have often clashed since, including over Caitlyn keeping her gender confirmation surgery plans secret.
"I didn't want them to leak it to the press, okay? And there was no reason for them to know about it," she added.