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12th August 2015
05:48pm BST
The couple married in July.
Della weighed in at 7 lbs, 6.5 oz and is Billy's first child with accomplished equestrian Alexis.
It was confirmed in April that the Uptown Girl singer was expecting his second child with 33-year-old Alexis.
The couple announced they were expecting in April.
A source told PEOPLE at the time: “Billy is thrilled to become a dad again.”
The couple tied the knot in a surprise ceremony in front of family and friends at their annual Fourth of July party this year.
Billy with his daughter Alexa Ray.
Billy (65) has a 29-year-old daughter, Alexa Ray Joel, from his marriage to Christie Brinkley.
Huge congrats to the family!
