Fire crews were called to the blaze at Rhostryfan, near Caernarfon, at about 19:00 BST on Saturday after a member of the public forced entry and discovered the property was smoke-filled.
North Wales Fire and Rescue Service said the bodies of a man and a dog were found in the ground-floor bedroom.
The cause of the fire is being investigated but North Wales Police said it was not suspicious.
A man and a dog have died following a fire at a house in Gwynedd.