The `MMMBop` singer - who already has a two-year-old son John Ira Shepherd Hanson with his spouse Kathryn `Kate` Tucker - made the surprise announcement on US radio show Kidd Kraddick.
He said: "I have a new baby. It`s coming. Nobody even knows."
When welcoming his firstborn in May 2008, Zac, 24, gushed: "There is not an audience or concert that could ever stand up to the feeling of meeting your baby for the first time."
Zac and Kate first met in August 2000 and married in 2006.
The musician`s latest arrival will add to the ever-expanding Hanson family.
Taylor Hanson and wife Natalie Anne Bryant have four children -Jordan Ezra, Penelope Anne, River Samuel and Viggo Moriah - while older brother Isaac has two children, Clarke and James, with wife Nicole.
Hanson - which was made up of the trio of brothers - first shot to fame in 1997 with their worldwide smash hit single `MMMBop`.