"It's been a few months, I've never seen her happier," people.com quoted a friend as saying.
In recent weeks, the two have been spotted out together in Brooklyn, where Williams lives and also in Los Angeles, where they enjoyed dinner at Comme Ca, on March 16.
The actor also tweeted: "If I fell in love would you guys be happy?"
"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," he added.
Williams has a daughter, Matilda, 6, from her relationship with Heath Ledger.