The new baby, whose name hasn't been revealed, joins brothers Kingston, 7, and Zuma, 5, Us Weekly reported.
Stefani, 44, announced her third pregnancy Jan. 17 on Instagram with a photo of herself wearing a golden crown.
"I was ready to hand over the crown. But I guess I am still queen of the house. #itsaboy #surroundedbyboys," she wrote on the caption.
This is the fourth child for Rossdale, 47. He has a 24-year-old daughter, Daisy Lowe, from a previous relationship.