Officers were called to the motorway near the Second Severn Crossing at about 14:10 GMT on Monday.
All eleven have been held on suspicion of immigration offences and checks are being carried out to establish their immigration status.
The driver of the vehicle has been arrested by Avon and Somerset Police.
Eleven people have been arrested after they and a baby were found on a lorry on the M4 in Monmouthshire, Gwent Police has said.