Michelle Williams and Jason Segel out in Brooklyn, NY. Credit: Jae Donnelly

No more blue Valentines for Michelle Williams!

The demure three-time Oscar nominee, 31, has fallen for longtime pal Jason Segel, a source confirms exclusively to Us Weekly. "They are smitten and very serious," the source says of the My Week With Marilyn star and Segel, 32. (The actress split from Heath Ledger five months before his death in 2008.)

The unexpected new couple was photographed taking a romantic stroll after a meal at Frankie's Spuntino in Brooklyn March 24. "He put his arm around her and made her laugh as they walked," says a witness. "[They] seemed very relaxed."

The next day, How I Met Your Mother star Segel was snapped bonding with Matilda, Williams' 6-year-old daughter, as she went for a ride on her scooter.

Friends for years through Williams' BFF and Segel's Freaks and Geeks costar Busy Philipps, their relationship only recently turned romantic.

They dined (and giggled) at L.A.'s Chateau Marmont Feb. 27, and then shared supper at West Hollywood's Comme Ca March 16.

Although they live on different coasts, the Brooklyn-based actress and L.A. denizen are "trying to make it work," the source says. "She hasn't been this happy in a long time."