Coleen announced the news on Twitter, telling her 840,000 followers that the baby is due in May. The couple already have a son, Kai, who was born in November 2009, reported Guardian online.
"A few people guessed, so news is out! Me, Wayne and Kai are thrilled that we are expecting a new addition to our family around May," Coleen tweeted.
She followed the message with a second one, saying, "It is still only early but bursting with excitement! Also wanted the news to come from us, before anyone else."
Manchester United and England striker Rooney and Coleen were childhood sweethearts and got married in a lavish ceremony in June 2008 when they were both aged 22.
A spokesman for the couple said, "Wayne, Coleen and their families are thrilled and delighted that Coleen is expecting their second child.
"They wanted to share the great news on their popular social networking sites and that is why Coleen tweeted the news ahead of any speculation. Everyone is thrilled for them."