Del Toro and Stewart welcomed the 8lbs 9oz child at an LA hospital over the weekend and have named her Delilah, a source close to the couple confirmed to People.
Another of the pair's close friends said that they are "thrilled" that Delilah is in good health.
Kimberly's father Rod Stewart has described his granddaughter's birth as one of the happiest moments of his life, saying: "My joy is indescribable."
Del Toro's rep first announced that Kimberly was expecting the Wolfman actor's child in April, but insisted at the time that the two were not a couple.
Delilah is the first child for both Del Toro and Kimberly.
Watch Benicio del Toro in the trailer for The Wolfman below: