The refugee, a 30-year-old man from Afghanistan, had been in the back of a truck travelling through Grande-Synthe, near Dunkirk in northern France.
"He had his head out the side of the truck's trailer and it must have hit an unidentified object," an emergency worker told AFP.
The man is understood to have died from the impact.
The truck driver heard the impact from the cabin of his vehicle, and pulled into a rest area and contacted police.
Other migrants in the back of the truck told officers that the deceased man had stuck his head out of the vehicle in an effort to determine where the truck was travelling.
Since June, at least 19 migrants have died in northern France in their efforts to get to the UK.
In the northern port city of Calais, some 4,500 people, mostly from East Africa and the Middle East, are camped in an improvised village known as the "jungle", hoping to reach Britain.