Deja-vu all over again!
Sienna Miller turned heads at Monday's Metropolitan Museum Costume Institute gala in NYC.
But it wasn't because of her stunning Emilio Pucci gown.
Her ex-fiance Jude Law was on her arm!
The two walked together on the red carpet -- their first major appearance since getting back together last year.
nx"They were holding hands and looked very happy in love," a witness tells UsMagazine.com. "Sienna was smiling ear-to-ear as she and Jude walked up the steps into the Met."
They weren't always so lovey-dovey: In 2006, they famously ended their two-year romance after Law, 37, admitted he had cheated on her with his children's nanny.
Miller, 28, declined to answer any questions about their on-again romance.
But she was more than happy to gush about her navy lace dress. Asked what she liked best about it, she told Us: "Everything!"