"Welcome Apollo Bowie Flynn Rossdale 2/28/14," Rossdale announced via Twitter Saturday. "Bowie and Flynn = [our] mothers' maiden names."
Rossdale, 48, and Stefani's new baby boy joins big brothers Zuma Nesta Rock, 5½, and Kingston James McGregor, 7½.
The couple kept mostly mum on the pregnancy until Stefani, 44, started her Instagram account off with a surprise in January, revealing her brood of boys would be growing by one.
"I was ready to hand over the crown. But I guess I am still queen of the house. #itsaboy #surroundedbyboys," she captioned a shot of her sporting a crown.