The pair were first spotted on a "romantic stroll" in March and were later said to be "very happy together".
In an interview with Extra, Segel seemingly confirmed the romance, but refused to give details.
"I think we would just very much like to be happy. That's all I'm going to say about it," the actor said while doing press for his new film The Five Year Engagement with co-star Emily Blunt.
Blunt added that Segel's relationship has changed him, joking: "He's a man-boy! He used to be a boy-boy and now he's a man-boy."
It was recently claimed that Williams is in love with Segel and smitten with their relationship.
> Jason Segel 'won't even cameo in Muppets sequel'
Watch the trailer for The Five Year Engagement below: