Hargitay, 47, married actor-writer Peter Hermann, 43, in 2004. They have a 4-year-old son.
"The couple are overjoyed by the wonder of their new daughter," Hargitay representative Leslie Sloane told UsMagazine.com. "Her brother, August, is thrilled to have the sister he's always wanted."
The celebrity news Web site said newborn Amaya Josephine was adopted in the United States.
Her birth date was not reported.
Hargitay is the daughter of late professional bodybuilder Mickey Hargitay and late actress Jayne Mansfield. She is best known for her work on "Law & Order: SVU," which shoots in New York.