The singer announced the baby news on her Instagram.com page on Friday.
She wrote: "I was ready to hand over the crown. but I guess I am still queen of the house. #itsaboy."
Stefani and husband Gavin Rossdale are already parents to sons Kingston, seven, and Zuma, five.
Rossdale, 48, also has a 24-year-old daughter, model Daisy Lowe, from a previous relationship.