Spears and singer-songwriter Demi Lovata will replace departing judges Nicole Scherzinger and Paula Abdul, who had confirmed their at the end of the first series they would not return for a new series.
Speaking at a press conference to launch the second series of the US primetime programme, Spears said: "I couldn't be more excited to join The X- Factor judging panel. I'm a huge fan of the show and now I get to be a part of the action. Simon and LA better watch out!"
Speculation suggested that Spears had been offered $15m (£9.4m) a year to take the role of judge to invigorate the show, which had suffered falling ratings in its first series.
The first outing of The X-Factor courted controversy from the off when popular US judge Cheryl Cole was axed from the judging line up before the first series was even broadcast, clearing a path for Pussycat Doll Nicole Scherzinger.
The Geordie Girls Aloud singer was reported to have "lacked chemistry" with fellow judge Paula Abdul, according to US media reports. Her strong accent is also believed to have concerned producers, who suspected the American audience might not understand her. However no official statements were made on either issue by the programme’s broadcaster Fox or production company Syco.
Lovato said: "I'm so thrilled to join this legendary group of judges. To be able to spend each week with Simon, LA and Britney watching the best that America has to offer is truly a dream come true."
The first series of the US X-Factor crowned Melanie Amaro as the eventual winner. In the UK the show is broadcast on ITV2.
In the UK the final of the last series of the 'The X Factor' pulled in 15.3 million viewers who tuned in to watch girl group Little Mix win. Viewers were four million down on the 2010 final.