"Hot mom(bod) summer," she captioned the photo of herself wearing a leopard bikini.
Geary's fiancé, musician Robin Thicke, commented on the post.
"Was I sleeping behind you?" he wrote.
Geary and Thicke welcomed their third child, Luca, on Dec. 11. The couple also shares two daughters.
"My Luca Patrick, you are so perfect," Geary wrote on Instagram when announcing the news. "Mommy & daddy love you so much."
Before becoming a parent, Geary had an abortion. She has been outspoken about her experience and admitted she attended therapy afterward.
"I knew I wouldn't be able to provide a good life for this child," Geary said on Instagram, according to People magazine. "It just wasn't the right time for me. I don't regret it."
The model also previously defended her decision after people criticized her for supporting abortion.
"I've been getting a bunch of DMs saying things like 'how could you support abortion when you're a mother yourself' so this is MY body, MY body after having an abortion in 2014, a miscarriage in 2017, delivering a baby in 2018 and 2019," she wrote at the time. "I'm glad I was able to make decisions about MY body without facing any type of punishment."