The 'No Doubt' hitmaker's newborn will join their two sons, Kingston, seven, and Zuma, five in the family, reported Contactmusic.
Stefani posted a picture of herself wearing a crown on Instagram and captioned it saying, "I was ready to hand over the crown. But I guess I am still queen of the house. It's a boy (sic)."
The singer did not inform when the baby is due, but many expect it to be within the next few weeks.
The 44-year-old star, who has a successful clothing range, had earlier said that she loves motherhood and watching her children grow.