The 37-year-old actress recently said "yes" to New York financier Andrew Youmans, according to Us Weekly.
"She wants a good guy and a stable family life," a source told the magazine.
Williams was spotted wearing a heart-shaped diamond ring on her left hand at the 2018 Golden Globe Awards this month.
She told People at the time that the ring was simply "beautiful jewelry."
Williams previously sported the ring at the Los Angeles premiere of "All the Money in the World" in December and during a vacation in the Bahamas with Youmans and Matilda, her 12-year-old daughter with late actor Heath Ledger, on Jan. 1.
Williams has kept her private life quiet since Ledger's death in January 2008 following their split.
She made headlines this month after USA Today reported she made less than 1 percent of Mark Walhberg's pay for "All the Money in the World" reshoots.
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