New couple alert: Michelle Williams and Jason Segel

Michelle Williams and Jason Segel ay be more than just friends.

According to Us Weekly, the longtime pals recently started dating and are already "smitten" and "very serious."

The proof seems to be in the pictures: The magazine has photos of the twosome hanging out last week in Williams' Carroll Gardens neighborhood with her 6-year-old daughter, Matilda, whose father is Williams' late ex, Heath Ledger.

The night before, the  "My Week With Marilyn" star, 31, and "Five-Year Engagement" funnyguy, 32, grabbed dinner at Frankies Spuntino, and on their way out, "He put his arm around her and made her laugh as they walked,"  a witness told Us Weekly, adding that "they seemed very
relaxed."

Though Williams lives in Brooklyn with her daughter and Segel is based in Los Angeles, the long-distance lovebirds are apparently  "trying to make it work," a source said.

Added the source: "She hasn't been this happy in a long time."