Williams’ daughter Matilda, 14, played matchmaker, the source told the publication.
Michelle and Thomas were spotted together in London where she is busy filming Venom 2.
Michelle was also spotted buying baby clothes at Seraphine's maternity boutique in Kensington.
Thomas directed Michelle in the drama Fosse/Verdon, for which she won the Emmy Award for lead actress this year.
Michelle was also previously married to Phil Elverum. The couple wed in 2018 and split less than a year later.
She has been candid in the past about finding love after Ledger’s death in 2008.
“I never gave up on love,” she told Vanity Fair in 2018. “I always say to Matilda, ‘Your dad loved me before anybody thought I was talented, or pretty, or had nice clothes'.”
She later told the Wall Street Journal she struggled to let go.
"At that time, I was inconsolable, because I felt, 'How will he be able to find us?'", she said. “This is where we lived, and he won’t know where we are,” she continued. “And now I can’t believe I thought that. Maybe that’s what’s making me cry is I feel sad for the person who thought he won’t be able to locate [us].”