Firefighters were called to the mid-terrace property in Ismay Street, Walton, on Monday night and found the man downstairs.
An investigation into the cause of the fire is under way, but police said there was nothing to suggest it was arson.
They are appealing for a group of youths who raised the alarm to come forward to help piece together events.
When fire crews arrived the ground floor of the property was on fire and the rest of the house was badly damaged by smoke, Merseyside Fire and Rescue Service said.
'Open mind'The man, believed to be in his 50s or 60s, was pronounced dead at the scene shortly after 2110 BST.
Det Ch Insp Neil Cowdrey, of Merseyside Police, said: "This was a serious fire in which a man has tragically died.
"We are keeping an open mind as to how the fire started but there is nothing at this stage of the investigation to suggest it was an arson.
"A resident has told us that some lads knocked on their door to say there was a fire and that resident was then able to alert the emergency services.
"We believe they have tried to help in some way and I would urge these lads to come forward and tell us what they know.
"They are not in any trouble and we are grateful for what they have done but their information could be crucial to solving how this fire started."
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