A U.S. magazine reports the former Dawson's Creek actress, 27, has started dating film director Spike Jonze, ex-husband of Lost In Translation director Sofia Coppola.
Williams and Jonze, 38, have been spotted together several times near the film-maker's £1.5million New York apartment in recent weeks.
Williams was devastated in January when her former fiancé Ledger died following an accidental drug overdose.
Since the actor's death, Williams has been struggling to stay strong for the sake of their two-year-old daughter Matilda.
But now it appears she finally has something to smile about again, thanks to a burgeoning romance with Jonze.
American magazine Star reports Williams was seen kissing Jonze outside his Lower East Side Manhattan apartment on July 2nd.
An onlooker told the publication: 'Michelle kissed Spike with a closed mouth on the corner of his lips.
'There was definitely a bit of caressing going on. She was clutching his arm. The body language was very romantic.'
Williams first met Jonze in 2006 when she auditioned for his film adaptation of the Maurice Sendak children's book Where The Wild Things Are.
She was offered a part, but later withdrew from the film.
Actress Chloe Sevigny, who also auditioned for the movie, said last year: 'I auditioned for Spike Jonze on Where The Wild Things Are, but Michelle Williams got the part. She's much prettier than me.'
But last year, the pair finally worked together on Synecdoche, New York in summer 2007 and struck up a friendship.
Just a few months later in September, Williams and Ledger ended their three year relationship.
During their time on the movie - which Jonze produced - Williams was still dating Ledger, while he has been romantically linked to Drew Barrymore.
After reportedly getting together this summer, Ledger's best friend Trevor DiCarlo, who has been helping Williams care for Matilda at her Brooklyn, New York, home, has given the new couple the seal of approval.
A source tells Star: 'I saw Trevor, Michelle and Matilda out for an afternoon in the neighborhood. They walked around chatting, went to a pizza joint for lunch and took the subway. Heath would be happy to know all his loved ones are doing so well.'