Mary-Kate Olsen's got a brand new man!

The 24-year-old has taken her new romance with Seth Campbell public, partying with the "chill" shoe designer at multiple New York City hotspots.

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"Their relationship is fairly new," a source tells of the couple, who celebrated a mutual friend's birthday Feb. 16 at NYC's Lavo and made out at a corner table all night. "They've been able to have a few dates on the down low. He's a downtown kind of guy, likes to party every now and then."

The couple also enjoyed a date night out at Mary-Kate's sister Elizabeth's birthday at The Jane.

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Adds the source of Campbell: "His company is in the press but he's not, so it works out that way. They're both like each other and are just seeing where things go. Having fun."

Olsen split from artist Nate Lowman in February 2010 after two years of dating.

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