The former Spice Girls singer will sit alongside Gary Barlow, Louis Walsh and Tulisa Contostavlos when filmed auditions begin later this week, according to The Mirror.
"Geri certainly isn't afraid to speak her mind and that's exactly what the show needs," a source told the paper. "She was in the Spice Girls, one of the biggest bands in pop history, so she knows exactly what it takes to succeed in this industry.
"She's a tough cookie and has dealt with everything the music business has thrown at her, so she's well qualified to spot contestants who have the talent and the personality to make it... no doubt her feisty personality will make for some brilliant television."
X Factor bosses are currently seeking a fourth permanent judge to complete the panel after Dannii Minogue reportedly declined an offer to return to the show last week.
Rod Stewart, Robbie Williams and No Doubt musician Gwen Stefani have also been linked with X Factor guest judging slots.
Halliwell previously stood in as a guest panellist two years ago when Minogue was pregnant with son Ethan, and was reportedly booed by audience members in Glasgow.
Filmed auditions for The X Factor begin on Wednesday (May 23) in Liverpool.