A Santa hat-clad thief has stolen a wheelchair from a man using a toilet in Federation Square.

The heartless crook left his victim stranded in the toilet for an hour until a passerby came to his aid.

Police are hoping to identify a man after a wheelchair was stolen and a man left stranded in a toilet at Federation Square in Melbourne.

The man struck up a conversation with the 54-year-old victim at Flinders Street station about 11am.

The pair went to a toilet at Birrarung Marr, where the offender assisted the man into the cubicle before stealing his electric wheelchair.