Richardson and his wife Kristin welcomed their son, Maxwell Haze Richardson, on July 10, the singer announced on his Facebook page.
The new arrival weighed 9 pounds and 8 ounces, and measured 23 inches long. Maxwell is baby No. 2 for the couple, who are already parents to son Mason, 6, and have been married since 2000.
The Backstreet Boys kick off their world tour next month in Chicago.