More than 40 men and boys, mostly Afghans, were found at the back of a refrigerated truck on a motorway in northern Greece.
A Greek police official said that the migrants were not injured and the refrigeration system had not been turned on.
Greek police say that the 41 people found were mainly from Afghanistan except for an Iranian and a Syrian, and six of the passengers were minors.
The truck, believed to have crossed over into Greece from neighbouring Turkey, was stopped during a routine check near the city of Xanthi. The driver was arrested, and the migrants have been taken to a nearby police station for identification.
The discovery comes just two weeks after the bodies of 39 people, believed to be Vietnamese migrants, were discovered at the back of a lorry trailer in Essex.