The NI Fire and Rescue service (NIFRS) were called to the Dungloe Bar in Waterloo Street in the city centre shortly after 6 am on Thursday, December 29.
Seven fire engines and 26 firefighters worked to put out the ground floor blaze.
NIFRS assistant group commander Kevin McLaughlin said that the cause of the fire was believed to be accidental.
“We believe the fire started in the kitchen,” Mr McLaughlin told BBC Radio Foyle.
“The premises was vacant and the fire was discovered when the cleaning staff turned up to begin work this morning.
“Most of the damage is to the rear of the premises. It started on the ground floor but penetrated up to the first and second floor.
“There is quite a bit of damage but, given the age of the building and the levels of compartmentation you would find in older buildings, I think our crews did well to minimise the damage to the area it was.”
Mr McLaughlin commended the firefighters from Derry and Strabane who dealt with the blaze in about four hours.
He said the NIFRS was examining CCTV footage in the area as part of its investigation.