The Fantastic Four stars are engaged, a rep for the actress confirmed to PEOPLE.
Mara, 33, was spotted sporting a stunning new sparkler in New York City this month, and showed off the same ring in a recent photo posted to social media.
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Bell, 30, was previously married to Westworld actress Evan Rachel Wood. The former couple tied the knot in 2012 before splitting after two years of marriage. Bell and Wood share one child, a 3-year-old son.