The child, Julia, vanished on Friday and was found lying in marshes several kilometres from her house on Saturday morning, with the dog by her side.
Julia is now in hospital in western Poland, suffering from frostbite after temperatures fell to -5C (23 F).
Firefighter Grzegorz Szymanski said the dog kept the child warm enough to live.
“For the whole night the animal was with the girl, it never left her. Remember, it was 5 degrees below zero and the child was wet,” he said, adding that the animal was the most important factor in the girl’s survival.
More than 200 people had searched for the child overnight. It is thought she spent hours wandering through the forest near her home in the village of Pierzwin.
Her parents had last seen Julia playing in the backyard with the small black mongrel.
Julia’s was eventually discovered by firefighters after she was heard crying for her mother.
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