The 28-year-old actress and Arnett were all smiles as they were spotted walking arm-in-arm in New York City recently, reported E! online.
Olsen and Arnett's romance sparked back in early February when they met while on vacation in Mexico.
An insider described Olsen's boyfriend as a "great guy," adding, "They are in an exclusive relationship and Lizzie is excited about him. She deserves it. It's very new though, but they seem to like each other a lot already."
Olsen's representative could not be reached for comment.
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