The 'I Want It That Way' singer's wife Kristin gave birth to son Maxwell Haze Richardson on Wednesday, 10 July.
Kevin announced the baby news on his Facebook page, saying: "Special delivery! Another Richardson boy... At 3:05am Wednesday morning (7/10/13), Maxwell Haze Richardson decided to join the party. He is a stout 9 lbs. 8 oz. / 23 inches long. Love! Love! Love!"
Thousands of fans from across the world liked the comment and left their congratulatory messages for the proud new parents.
A message on the official BSB Twitter account said: "HUGE Congrats to the Richardson family!! We've got a new Backstreet Boy on our hands :) Can't wait to meet ya Maxwell!"
Kevin and Kristin, who announced the pregnancy at Easter, are already parents to six-year-old son Mason.
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Copyright: Love & Lust London