And there's pictorial evidence, folks. Check out a snap of their stroll here.
"He put his arm around her and made her laugh as they walked," a source tells the magazine. "[They] seemed very relaxed."
The next day, Segel was photographed with Williams' six-year-daughter, Matilda, as she rode her scooter.
They pair also dined at Hollywood's Chateau Marmont on February 27 and then shared supper at West Hollywood's Comme Ca on March 16, according to Us Weekly.
According to the mag, the two have been friends for years and were introduced by Williams' good friend Busy Philipps. She also starred with Segel in TV show Freaks and Geeks.
And despite living on different coasts - the My Week With Marilyn star is rooted in New York, while Segel is still based in Los Angeles - they are trying to make it work according to a source, who added: "She hasn't been this happy in a long time."
According to E! News, earlier this month Segel asked his Twitter followers "a totally hypothetical question" to find out, "If I fell in love would you guys be happy?"
It was met with a resounding YES from his disciples.
"We don't even know each other and you guys want me to be happy," he reportedly replied. "I'm not being sarcastic at all when I say that actually means a lot."
All those tweets have subsequently been deleted.
Segel recently told Parade magazine that he doesn't believe in "the one".
"I think relationships are an effort and it's about timing, an actual decision, and two people deciding they will be the best person for one another," he said.
"And what happens if you miss the one or they die? Now you are doomed to a life of loneliness? We know that's not true."
He added: "I don't want to have a girlfriend for the sake of having a girlfriend. I have no desire to pick a random girl. I want the right girl. There are plenty of opportunities, but I would rather have the right one."
Is Williams the one? Stay tuned ...
Check out the trailer for My Week with Marilyn below:
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