Williams and Ledger, who co-starred in the film "Brokeback Mountain," are the parents of 2-year-old Matilda Rose.
Williams issued a statement through her spokesperson, who also represented Ledger, MTV.com reported.
It was the first time she has publicly remarked about Ledger's Jan. 22 death in New York.
"Please respect our need to grieve privately," Williams said in the statement.
"My heart is broken," she added. "I am the mother of the most tender-hearted, high-spirited, beautiful little girl who is the spitting image of her father. ... All that I can cling to is his presence inside her that reveals itself every day. His family and I watch Matilda as she whispers to trees, hugs animals and takes steps two at a time, and we know that he is with us still. She will be brought up with the best memories of him."
The cause of Ledger's death has not yet been determined.