The 37-year-old singer is understood to be standing in for pregnant Dannii Minogue at auditions in Glasgow today.
Halliwell was previously a judge on Popstars: The Rivals, which discovered X Factor judge Cheryl Cole.
Geri Halliwell spices up The X Factor
GERI Halliwell is expected to add some extra spice to The X Factor by stepping in as a guest judge.