According to Caplan's rep, the two have actually been engaged for a couple of months; Riley, 35, popped the question in May in New York City.
Reports of the engagement first surfaced on Friday morning, when the Masters of Sex star was spotted wearing a ring on that finger while grocery shopping with Riley in Los Angeles, even though Caplan did wear the same ring to the Ghostbusters premiere earlier this month, as well as the premiere of Now You See Me 2 in June.
The pair first met while Caplan was filming in London in January 2015, confirming their relationship a year later as they arrived as a couple to the 2016 Prague Opera.
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While there's no word yet on when they'll tie the knot, it seems the 34-year-old actress has already got an idea of who’s going to raise any future children.
"I actually romanticize the idea of a stay-at-home dad. Partly because I think that's a very sexy thing, but also because if there are two working actors and both of them get a job and they have a baby, the woman is going to stay with the baby. I see it all the time. It just seems unfair," she told Elle Canada in 2014. "I don’t want it to be an assumption that I will be the one to stay home and raise the kids."