Segel and Williams were seen going for 'a romantic stroll' after dinner in Brooklyn on Saturday (March 24), reports Us Weekly.
"He put his arm around her and made her laugh as they walked," a witness claimed. "[They] seemed very relaxed."
The next day Segel, who has apparently been on a couple of dates with the My Week with Marilyn actress, was photographed hanging out with her 6-year-old daughter Matilda as she rode her scooter.
A source added: "They are smitten and very serious."
Segel and Williams are said to be long-time friends after meeting through actress Busy Philipps, who is Williams's best friend and who starred with Segel in NBC's Freaks and Geeks.
"She hasn't been this happy in a long time," a source claimed.
Watch the trailer for My Week with Marilyn below: