Jason Segel is "totally smitten" with Michelle Williams.

The couple have only been dating for three months but pals of the How I Met Your Mother actor say he has never felt this way before.

"Jason is totally smitten. It's like the first girl he's ever been this way with," a source said. The insider added that Michelle - who has a six-year-old daughter Matilda from her relationship with the late Heath Ledger - "hasn't been this happy in forever."

Recently, the pair enjoyed a "lovey-dovey" date as they spent a romantic evening watching James Corden's One Man, Two Guvnors Broadway show at the Music Box Theatre on West 45th Street, New York. "They look very lovey-dovey. She looks so pretty in white, and he's in a suit," an audience member said.

Previously, the blonde beauty was seen leaning in and kissing Jason on the cheek and rubbing his back at the after-party of the premiere of his latest movie The Five Year Engagement in April. The 32-year-old actor was also spotted holding the 31-year-old star's hand and whispering in her ear at the New York event.