Caitlyn Jenner and William Behr. Picture: Instagram

Caitlyn Jenner has become a grandparent for the sixth time and announced the news on Instagram Sunday evening.

The 69-year-old star shared an adorable photo of herself with little William Behr - the son of Caitlyn's oldest son Burt and his partner Valerie Pitalo - on her Instagram account on Sunday, February 17, a week after the tot was born on February 9.

Caitlyn captioned the post: "Welcome to the world William Behr Jenner. Congratulations @burtonjenner and @valeriepitalo !! Love you guys"

It has been suggested the baby took his first name in honour of Caitlyn, who was born William Bruce Jenner.

Burt announced the arrival of his second son by sharing a cute video of his eldest child, two-year-old Bodhi, cuddling his baby brother and declaring him to be "nice".

As well as Bodhi and William, Caitlyn is also grandmother to her daughter Kylie Jenner and Travis Scott's 12-month-old daughter Stormi, her elder daughter Cassandra and husband Michael Marino's three kids, Francesca, 10, Isabella, seven, and Luke, two, and son Brandon Jenner and his ex-wife's three-year-old daughter Eva.