The politician's staff had begun to suspect a poltergeist was loose in his three-storey home in Streatham, London, after objects began moving from room to room.
It was only when the teetotal politician found an empty bottle of gin and a gay video in his closet they realised another person was living in the property.
Mr Galloway said that he first noticed something strange was going on when all his ties were moved out of his wardrobe.
Other staff said shoes they had left under beds mysteriously disappeared and shaving equipment was found to have been disturbed.
The politician said: "I have a sword, given to me in Saudi Arabia or somewhere, so I unsheathed my sword, and went upstairs [to the top floor].
"There was no-one there, but there was a bottle of gin – which, of course, would never be in my house – and a gay video, which definitely would never be in my house. So the police came, and they said this person appears to have been living here in your house for some time."
Detectives said there had been a spate of break-ins in the area, where homeless people got into people's homes when they were away and started bedding down. They were able to find fingerprints and footprints on a downstairs window which the intruder had been using to get in and out.
The incident happened in 2010, when Mr Galloway was in Egypt for a time. However, his house is used by members of his staff and contains his office, meaning several people were coming and going at the time the homeless person was living on site.
Mr Galloway bought the house, one of several he is said to own, from the proceeds of his various lawsuits against newspapers he sued for libel and defamation.
He has now fitted the house with a number of devices to ensure no more secret lodgers set up home.
He said: "No one can do it now because I've got CCTV and alarms and so on."