The 36-year-old Jason Bourne actress shared a photo of herself with Strummer, who is seen in a baby carrier with his face turned away from the camera.
"I haven't worn a back pack since middle school," Stiles wrote in the caption. "Now I have a front pack. #jujube #tulababycarriers."
Stiles waited until Nov. 20 to announce Strummer's birth, a month after he was born. She posted a photo of his tiny hand holding her thumb.
"Strummer Newcomb Cook, born October 20, 2017," she wrote in the caption. "Thanks and ever thanks to the extraordinary Doctors, Nurses and staff at Mount Sinai for helping bring this utter joy into our lives. 'Hello, World!'"
As Entertainment Tonight reports, Stiles and her husband, Preston J. Cook, married in September. She celebrated the wedding with a photo from the wedding, with Cook's hand on her stomach. "Who doesn't love a shotgun wedding?" she wrote.
Cook and Stiles met while he was a camera assistant on Blackway, which they made in 2015. It wasn't released in the U.S. until 2016.
Stiles is best known for her role as Nicky in the four Matt Damon-starring Bourne movies. She also starred in 10 Things I Hate About You, Silver Linings Playbook and Dexter, which earned her a Golden Globe and Emmy nomination.
Photo: Julia Stiles/Instagram