An investigation was launched after flames tore through Mr Earl's Sports and Social Club, on Earl Street, Higher Ince, on Sunday night.
A fire service spokesman said it was believed the fire had been started deliberately and police confirmed they were investigating.
Firefighters were called to reports of the blaze at 8.45pm and five crews, from Wigan, Hindley, Atherton and Leigh fire stations, worked to bring it under control.
But part of the single-storey building collapsed and witnesses reported seeing a large hole in it.
A fire service spokesman said: "Unfortunately the building has suffered a partial collapse."
The building was once home to the Higher Ince branch of the Royal British Legion.
But the organisation moved out more than 20 years ago and members found a new home elsewhere, before disbanding entirely in 2008.
It became Mr Earl's Sports and Social Club in 1995 and continues to operate there.
The Royal British Legion put the site up for sale in 2017, but the manager of Mr Earl’s said at the time that the club was continuing to operate and had a 99-year lease.
A planning application was submitted in the summer seeking permission to demolish the club and build 38 houses there.
A decision on that application has not yet been made, according to Wigan Council's website.