Security guards reported to police about 8.50am that the Hard Rock neon sign was on fire.
''It started with the big H and spread from there,'' a witness told Phuketwan at the scene. Patong police are investigating but an electrical fault is suspected.
A spokesperson said it was too early to assess damage to the building, but fortunately there was no inside damage.
''We could even open as usual tonight with permission from the local authorities,'' he said. Sales and marketing manager Sasichol Thongnak was more cautious.
''Insurance assessors will need to look at the damage and safety will obviously be a concern,'' she said. ''But at the most, I'd say we could be closed for two or perhaps three days, no longer.''
The outside of the building, on Rat-U-Tit 200 Pi Road, was badly blackened. However, the iconic large guitar - destroyed according to early reports - survived.
The Hard Rock Cafe occupies the end of the old Courtyard by Marriott Resort and is a popular destination with tourists and locals.
The guitar is a landmark at that part of Patong.