The victim was hunting moose Tuesday morning near Swan Hills, Alta., said RCMP Const. Graham Bourque.
"A young cub walked by him while he was out moose hunting with his bow and arrow and the cub saw him, got spooked, ran up a tree, started whining and we believe the mother went after him," he said.
The grizzly attacked the hunter from behind, but he managed to fire a round from his gun, scaring the bear off.
The hunter later said he never saw the bear coming.
The injured hunter was forced to hike five kilometres before he could find an area with cell phone coverage.
When rescuers arrived on ATVs, the hunter was conscious, but had serious injuries to his head and legs, said Bourque.
The man was transported to Grande Prairie hospital by air ambulance where he is in stable condition.
Swan Hills is more than 200 kilometres northwest of Edmonton.