Little Matilda was born Friday in Brooklyn, where the engaged couple recently purchased a $2.6 million home, Zap2it.com reported Tuesday.
Ledger, 26, and Williams, 25, met on the set of Ang Lee's gay cowboy movie, "Brokeback Mountain."
Australian Ledger will next been seen in "Casanova," which opens in November. His other credits include "10 Things I Hate About You," "The Patriot," "A Knight's Tale," "Monster's Ball" and the "Brothers Grimm."
Williams is best known as Jen Lindley on the WB's "Dawson's Creek."