A source tells Us that the couple, who have remained extremely private about their relationship, plan to tie the knot this year.
Mara, 33, and Bell, 30, got together after meeting on the set of 2015's Fantastic Four, in which they played Sue Storm and Ben Grimm, respectively.
The House of Cards alum and Bell first sparked dating rumors when they were spotted kissing in NYC's Soho neighborhood in October 2015. Last week, Mara revealed her diamond ring in an Instagram photo.
In August 2015, Mara and Bell made an appearance on Watch What Happens Live with their Fantastic Four costars Miles Teller and Michael B. Jordan. During the Plead the Fifth segment, Mara was asked who she would "Marry, Shag, Kill" out of the three men.
"I'm going to marry Jamie," she gushed at the time to host Andy Cohen.
This will be the first marriage for Mara and the second for Bell. Us exclusively confirmed in May 2014 that he and Westworld's Evan Rachel Wood split after nearly two years of marriage. The pair welcomed a baby boy in July 2013, but have not revealed his name.
E! News was first to report the news.
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