The Take That singer - who has son Elwood Jack, six, and daughter Willow Rose, four, with wife Emma Ferguson - revealed the happy news after his spouse`s bump became too big to conceal when she passed the four-month mark, and is said to be "over the moon" at becoming a dad again.
A friend of the couple said: "He`s over the moon. He was keeping it a secret for as long as possible so he could enjoy keeping the special news in the family. But now it`s come to a point where it`s time to share it with everyone."
Mark`s announcement marks a trio of baby news for Take That, with Gary Barlow and his wife Dawn currently expecting their fourth child and Robbie Williams revealing last week that he and wife Ayda Field are to become parents for the first time.
The source added to The Sun newspaper: "The lads have had a right old giggle about the three of them having baby news about the same time.
"Gary was joking there must have been nothing on the telly a few months back for them all to get down to business."
The 40-year-old singer`s marriage hit a rocky patch in 2009 amid claims of him cheating on her, but the couple now believe life "couldn`t be better".
An insider explained: "Mark and Emma have gone through a lot of soul-searching.
"Mark`s turned his life around and proved to Emma that he is the man she fell in love with. Life really couldn`t be better for them."