The "nighthawks" (that's the term for illegal metal detector-wielding treasure hunters) are targeting the Brunton Turret section of the 1,900-year-old World Heritage Site, an official of Historic England said.
The turret and wall section were built by the men of the Twentieth Legion of the Roman Army.
They are registered monuments where using a metal detector without proper authorisation is a criminal offence.
The ruins of the Brunton Turret section are surrounded by further buried archaeological remains from the frontier of the Roman empire, which are very vulnerable to damage from nighthawks, Historic England said.
Historic England is calling on visitors to Hadrian’s Wall and Tyne Valley residents to report illegal metal detecting which is "causing loss and damage to our shared cultural heritage."
Mike Collins, Historic England's inspector of ancient monuments at Hadrian's Wall, said: "We know that the majority of the metal-detecting community complies with the laws and regulations regarding discovery and recovery of objects from the land.
"But the small number of people who steal artefacts and damage ancient sites are breaking the law and robbing us all of the knowledge and understanding that objects from the past can give us."