The soccer star and his wife revealed the news after having a scan yesterday (20.12.12), confirming the sex of the child.
Coleen, 26, tweeted: "Absolutely loved seeing our baby on the scan yesterday!!! We decided to find out the sex...... It`s a ..... BOY!! So exciting (sic)."
The couple`s second son will be a brother to Kai, who turned three in November.
Coleen announced her pregnancy in October, when she told her twitter followers: "A few people guessed, so news is out ha!! Me wayne and Kai are thrilled that we are expecting a new addition to our family around May time x (sic).
"It`s still only early but bursting with excitement! Also wanted the news to come from us, before anyone else! (sic)"
Coleen has previously admitted she would love to have a large family, saying: "I`d like to have maybe three or four children. I`d be happy to be a full-time mum."
Manchester United ace Wayne, 27, and Coleen married in Italy in 2008 after a five year engagement and he was at her bedside as she gave birth to Kai.
A source said at the time: "Wayne was a star with Coleen. He didn`t leave her side from the moment she came into the hospital.
"When Coleen gave birth, Wayne`s eyes welled up. As soon as he was handed the baby, the tears began."