"Welcome Apollo Bowie Flynn Rossdale 2/28/14," Rossdale tweeted on Saturday, confirming the bundle of joy arrived on Friday. He added, "Bowie and Flynn = mothers' maiden names."
Apollo joins big brothers Kingston, 7, and Zuma, 5. The Bush frontman is also dad to model Daisy Lowe, 24, from a prior relationship.
"I was ready to hand over the crown. but I guess I am still queen of the house. #itsaboy #surroundedbyboys," the three-time Grammy winner, 44, wrote in the accompanying caption.
The L.A.M.B. designer celebrated her baby shower with a star-studded bash on Feb. 8. The blond beauty was joined by close family members as well as Gwyneth Paltrow, Nicole Richie, Rachel Zoe, Jessica Alba, Kate Beckinsale and Chelsea Handler, a source told E! News at the time.
According to an eyewitness, Stefani looked "radiant" as she arrived to the party, sporting black leather pants and a colorful blazer.
"As she walked over the bridge and joined her friends she had a big smile on her face and was giving everyone hugs," the eyewitness said. "There was a big welcome cheer for her and she seemed very grateful for all of the love."
Stefani shared a few pics from the baby shower, revealing a table full of blue-iced doughnuts and blue candies, gorgeous white roses surrounded by candles, a decadent slice of marbled cake and a baby's head cake.
Us Weekly was first to report the exciting news.
Congrats to the new family!