The House of Cards actress and Billy Elliot actor welcomed a baby girl together "a couple weeks ago," but just now broke the news. This is Mara's first child, and Bell's second, as he has a son with ex-wife Evan Rachel Wood.
Mara debuted a shot of the baby's feet, but kept the child's face out of frame.
"We had a baby a couple weeks ago," Mara wrote. "Here are her feet."
The photo has been liked more than 13,000 times in the hour since it was uploaded.
Mara and Bell also received some warm congratulations from famous friends and fans in the comments of the photo.
"Awwww so sweet!!" Friends actress Courteney Cox wrote. "Congratulations you two!!"
Author Caitlin Cronenberg wrote, "CONGRATULATIONS!!! Beautiful feet!!!"
Makeup artist Grace Lee added, "Ohhhhhhhh congrats!!! Her little feet are just perfect. Welcome to motherhood!!"
The couple while filming the 2014 Marvel superhero film Fantastic Four, with Mara playing "Invisible Woman" Sue Storm and Bell playing "The Thing" Ben Grimm. They became engaged in January 2017 and wed by July 2017.
Bell will be next seen in the Elton John biopic Rocketman as songwriter Bernie Taupin. Ahead of the film's release, Bell discussed fatherhood during an interview with ES Magazine. He opened up about how his own father left him as a child, understanding how one might be compelled to leave their family behind.
"It's difficult raising a child," he said. "It's really stressful. It requires a lot of you. You have to sacrifice your life and some people don't want to do that. It's not particularly noble, but I could see why the easier thing is to leave."
He clarified that he had no plans of going anywhere after the child's arrival, saying, "F— no. You can't be thinking, 'I'm doing this because no one ever gave me this experience.' The thing is not to love from a place of anger because that is potentially devastating."
Photo Credit: Dave M. Benett/Getty Images