The Backstreet Boy, who wed his model girlfriend Rochelle DeAnna Karidis in December 2011, announced the news on YouTube.
"I just wanted to say thank you guys for all your love and support. We recently got married - I was the most recent Backstreet Boy to take the plunge," McLean said.
"Because we love you all so much, we wanted to share with you - for the first time - something very special. We're having a baby!" he finished.
The Backstreet Boys have said that they will record a new album this year.
McLean postponed his wedding to enter rehab in January 2011.
Watch AJ McLean make the announcement about his baby below: