A rep for the actress confirmed to CNN that it's a boy.
The "Orange Is the New Black" actress announced the happy news on Instagram, with a post of her and her son.
"Welcoming home our new bundle of love. Overwhelmed with gratitude," Prepon captioned it.
The baby joins the couple's 2-year-old daughter, Ella.
A number of celebrities commented on the post, with Natasha Lyonne posting heart-eye emojis over the news and Chelsea Handler writing, "Happy birthday to mommy and baby!! Love you."
Prepon, who previously starred in the sitcom "That '70s Show," announced her pregnancy back in October, writing on Instagram, "We are so excited to announce that our family is growing. Life is beautiful!" and recently annoucing that she was soon due and would be going on maternity leave.
Prepon and Foster, also an actor, got engaged in 2016 and married in June 2018.
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