"Jack Leon Lynch is born! Born at 7:06 p.m. tonight and we are both doing well," the new mom, who gave birth in Belgium, writes. "His big sister, mama and daddy are very proud and happy!"
Baby boy joins the couple's daughter Jada Ellie, 5½.
Clijsters, who retired from the sport after competing in the 2012 U.S. Open, announced her pregnancy in February.