The "Oops! ... I Did It Again" singer, 22, married Federline during a Sept. 18 ceremony in a Studio City home, but didn't immediately file the license with the county, raising questions about whether they were legally wed.
Kathy Tacawy, a county registrar's spokeswoman, confirmed the documents were filed in Los Angeles County within the 10-day legal window. She said the marriage was official, but declined to comment further and wouldn't say when the documents were filed.
A Spears' spokeswoman had previously said the couple had filed the marriage license, but county officials hadn't confirmed it.
Federline, 26, has said the delay in filing the paperwork was caused when the couple moved up their wedding date as word leaked out that they were planning to be married on Oct. 16. Moving the date up a month, he said, didn't give their lawyers time to finish their prenuptial agreement.
In a recent posting on her Web site, Spears wrote: "This is the happiest I've ever been in my life. ... We are just settling into our new home and it is coming together perfectly.
"I'm redecorating a lot of things because when we decide to start a family, everything in it needs to be perfecto!"
Her new album, "Britney Spears Greatest Hits: My Prerogative," was released Nov. 9.
In January, Spears married childhood friend Jason Alexander in a surprise wedding in Las Vegas. That marriage was annulled 55 hours later.
Federline was previously involved with actress Shar Jackson of TV's "Moesha." They have two children — a 2-year-old and a newborn.