The 38 year old and her boyfriend, fellow actor Hamish Linklater, welcomed their second child together in June (20), Rabe's representative confirms to People.com.
No further details have been revealed, but the news emerges days after the new mum revealed her recent pregnancy in an Instagram post for the platform's Throwback Thursday trend.
In the photo, Rabe stood in front of a wall wearing a pink dress and a face mask, as she rested one hand on her large baby bump.
"Looking back at a day in May," she captioned the snap. "#wearamask #wearad**nmask #tbt."
The couple also shares a daughter, born in early 2017, while Linklater is additionally dad to 13-year-old Lucinda Rose from his marriage to screenwriter Jessica Goldberg.