"At around 7:30am (0530 GMT), 31 people were found inside a refrigerated heavy goods vehicle in a layby on the A16," police said.
The discovery was made near Dunkirk on France's northern coast.
A three-year-old child was among the group. All but one of them were described as "safe and sound" despite the fact the temperature inside was "less than a degree" (under 34 degrees Fahrenheit), police said.
It was not immediately clear how long they had been inside the truck.
Most of the group said they were Syrian nationals but one was Vietnamese, police added.
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The driver, a Spanish national, was apparently unaware of their presence, telling police he had pulled over when he heard "noise coming from his trailer".
He said the lock of the truck had been forced and, unable to open it, had called the police.
Thousands of migrants have headed towards France's northern coast in the hope of finding passage across the Channel to England.
On Tuesday, a 20-year-old man was found dead inside a lorry near the port of Calais, crushed to death by wooden pallets.
And the evening before, 15 migrants were found inside a tanker in Arras, some 95 kilometres (60 miles) further south, where they had been in contact with a toxic substance containing lead, although none of them showed any adverse symptoms.
On August 27, the decomposing bodies of 71 people were found inside a truck at the side of an Austrian motorway in a discovery which sparked a horrified response across Europe as it struggles with its worst migration crisis since World War II.