According to a report in US Weekly and television show Access Hollywood, the baby was born Wednesday by Caesarean section at the Santa Monica UCLA Medical Center in California. No other details were offered.
A call to Spears' publicist by The Associated Press wasn't immediately returned.
If the pop star has indeed crossed over into motherhood, it's a joy she isn't yet ready to share with her fans. The latest news offered on the singer's official web site, BritneySpears.com, consists only of a plug for her new perfume, Fantasy (arriving in stores Thursday, she says), a note of condolence to hurricane victims assuring fans that she and her relatives in Louisiana are all safe, and a photo of her dog at an August baby shower.
Perhaps that last posting offers a hint.
This would be the first child for the 23-year-old pop star and her husband, Kevin Federline. Federline, 27, has two children with ex-girlfriend Shar Jackson.
Spears, 23, and Federline announced the pregnancy in April.
Reports suggest that they will call the baby either London Preston or Preston Michael. Her top name for a girl was Addison Shye.
Last month, the singer revealed: "I have a feeling I'm going to have an operation. I don't know why but I hope so.
"My mum said giving birth was the most excruciating thing she's ever gone through in her life. So if a caesarian doesn't happen, I'll be like, 'Epidural, please'."
The Toxic star, who has sold more than 60 million albums, has made no secret of her wish to start a family and has said she can see herself as a mother.
Shortly after tying the knot in a private ceremony last September, she announced she would be taking a break from music to enjoy married life and focus on motherhood.