Historic England says nighthawks, as illegal metal detectorists are known, have dug more than 50 holes at the Brunton Turret section of the 1,900-year-old World Heritage Site near Chollerford in Northumberland.
As well as damaging the ruins of the turret, they are also breaking the law, as it is illegal to use a metal detector without permission at a scheduled monument.
Similar nighthawking incidents have affected Corbridge, Housesteads and Steel Rigg, other sections of Hadrian’s Wall, over the last three years.
Brunton Turret is one of the best-preserved turrets on the wall, with a stretch of wall 64 metres (210ft) long, and was built by the twentieth legion of the Roman army.
Historic England said the activity is “causing loss and damage to our shared cultural heritage”.
Mike Collins, the agency’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.
“These nighthawks are committing a criminal offence and we’d like everyone’s help to ensure they are caught.”
Mr Collins said the majority of items taken were sold on, either via online auction sites or to collectors.
Mark Harrison, head of heritage crime and policing advice for Historic England, said: “Illegal metal detecting is not a victimless crime.
“We may never see or fully understand the objects taken or damaged because they have been removed from their original sites with no care or record as to their history or context.”
The 73-mile (117km) wall stretches between Wallsend in North Tyneside and Bowness on Solway in Cumbria and has about 160 scheduled monuments, including Roman camps, forts and signal stations.