Michelle Williams is reportedly dating Jason Segel.

The My Week With Marilyn actress - who has a six-year-old daughter, Matilda, with late ex-fiance Heath Ledger - is said to have seen her long friendship with the Muppets star grow increasingly serious in the last few weeks.

"They are smitten and very serious," a source said.

The pair were spotted taking a romantic stroll in New York after a meal at Frankie's Spuntino on Saturday. "He put his arm around her and made her laugh as they walked. They seemed very relaxed," one onlooker told Us Weekly magazine.

The following day, 32-year-old Jason was seen bonding with Michelle's daughter as she went for a ride on her scooter. While Michelle and Matilda reside in New York and Jason is based in Los Angeles, the new couple are hopeful they can make their relationship work.

"[They are] trying to make it work. She hasn't been this happy in a long time," the source said.

Michelle split from Heath five months before he died of an accidental drug overdose in January 2008. Since the actor's passing, she had a year-long relationship with director Spike Jonze which ended in 2009.