The 44-year-old singer and Rossdale, who are already parents to seven-year-old Kingston and five-year-old Zuma, named their tot Apollo, reported Ace Showbiz.
"Welcome Apollo Bowie Flynn Rossdale. 'Bowie' is the maiden name of Rossdale's mother while 'Flynn' is Stefani's mother's maiden name," Rossdale tweeted yesterday.
Stefani revealed in January that she was pregnant with another boy.
Posting an image of her in blue T-shirt and crown, she wrote, "I was ready to hand over the crown. But I guess I am still queen of the house. Its a boy. Surrounded by boys.