Mara and Bell worked together in Fantastic Four in 2015, in which they played Sue Storm and Ben Grimm, respectively, but they met long before they were costars.
Kate has confirmed that the pair, who have been dating for two years, said "I do" in an intimate ceremony. She captioned the post, "Nuptials".
"We fell in love on the press tour", Mara revealed about their off-screen romance.
The Fantastic Four co-stars got engaged in January.
This is the first marriage for the former "House of Cards" actress and the second for Bell, who was previously Wednesday to actress Evan Rachel Wood, with whom he has a child. Bell, 31, was previously married to "Westworld" star Evan Rachel Wood and has a 4-year-old son from the relationship.