Jenner said she had rented the helicopter for Dream, her niece, for her third birthday party in November.
This was the helicopter that crashed on Sunday, which killed basketball star Kobe Bryant, his daughter, Gianna, 13, and seven other people.
Along with a picture of Kobe and Gianna, Jenner posted an Instagram Story, writing: "rest in peace..and prayers to these families. i still can't believe this."
"At loss for words right now. Praying for this beautiful family," Jenner wrote.