The 26-year-old actor popped the question with a pink gold and diamond ring, while the couple were in New York City over the festive period, according to Extra.
Emma and Evan jetted off to London to ring in the New Year, and returned to New Orleans on Sunday. Speculation first started that the lovebirds were engaged when Emma was spotted flashing the diamond ring as she landed at the airport stateside.
The news comes just six months after Emma was arrested for reportedly attacking her beau in a Montreal hotel last July. The rising star allegedly left Evan with a bloody nose after an incident of "domestic violence".
Emma was not charged with any crime after Evan dropped the charges, and hours after news of her arrest broke, she was pictured weeping in her boyfriend's arms.
"Emma is very dramatic, she gets wound up during arguments," a source told Us Weekly of the couple's relationship. "When they are good, they are crazy in love.
"Their romance is pretty extreme. They just behave in a way that's very passionate."
The American Horror Story co-stars first met on the set of dark comedy film Adult World, but didn't always get on. Months after, the pair started dating in 2012.
"I come from the school of getting married once," Julia Roberts' niece has previously said. "Every relationship should be important. Everyone kind of rolls their eyes at me, but I still believe in the romantic movie outcome."