Fire gutted the Rhiw Goch Inn, situated on a holiday complex near the Snowdonia National Park, in the early hours of Sunday 14 October.
Nobody was injured in the blaze as it occurred when the pub was closed, and emergency services prevented the fire spreading to nearby buildings.
The fire started at around 12.30am and required six fire-fighting appliances, two water bowsers and an aerial ladder platform to tackle the blaze.
North Wales Fire and Rescue Service has launched a joint investigation with North Wales Police.
The site was originally built as a farmhouse in the 12th century and was a childhood home of the saint John Roberts, who was martyred following the Protestant Reformation.
It was a training camp for the artillery arm of the British army in the early-20th-century before it became a hotel and restaurant in the 1970s.
Kevin Jones, community safety manager for the Conwy and Denbighshire public service board, said: “The cause of the fire, which destroyed 100% of the historic building, is under investigation.
“Thankfully no-one was injured and the efforts of the fire fighters in attendance prevented the fire spreading to adjacent buildings.”
Inspector Jason Higgins, of North Wales Police, said: “We believe a number of students were staying at the location at the time and we would be keen to hear from them.
“We are also appealing to anyone who may have filmed the fire on their mobile phone to contact police on 101 quoting reference W146588.”
Inn management pleaded for anyone with information on the incident to contact their Facebook page.
Liz Saville Roberts MP, the Westminster leader for Plaid Cymru, tweeted the fire was a “great loss to Wales and Gwynedd”.
The nearby Trawsfynydd Holiday Village informed its visitors of the fire on its website.
A notice read: “It is not run or owned by us and our thoughts are with the owners at this difficult time. We hope that once the shock has subsided they will consider rebuilding the business up in the future.”