PDA Alert! Sienna Miller & Much Younger BF Lucas Zwirner Make Relationship Official

Sienna Miller, 37, and boyfriend Lucas Zwirner, 27, were spotted packing on the PDA in NYC!

The couple confirmed their relationship with a romantic stroll as they held hands, shared kisses and snuggled up together on a bench.

She wore jeans, a khaki jacket, green beanie, gold earrings and leopard print boots, while he bundled up in an orange winter coat, blue beanie and white sneakers. It was a casual day out for the pair as they walked a black and white dog.

Sienna and Lucas were first spotted together a few weeks ago when they attended her ex-fiancé Tom Sturridge's birthday party in London.

After Tom's party, Lucas was labeled a mystery man. It has since been learned that he is a Yale grad who works for his famous father, David Zwirner, a powerful art dealer who opened the David Zwirner Gallery in 1993 in NYC.

W magazine reports Lucas is the editorial director at David Zwirner Books, the gallery's publishing house, and he oversees 25 book releases per year.