Archaeologists from Glasgow and Exeter, studying images gathered during covert intelligence operations in the last century, have identified a wall that ran around 37 miles from the Danube to the Black Sea over what is now Romania.
Built in the mid-second century, the barrier once stood 28ft wide and around 11.5ft high. It is thought to have served a similar purpose to other Roman frontiers such as Hadrian's Wall, built to defend the Roman Empire from threats to the borders.
Known locally as Trajan's Rampart, it consists of three separate walls which were wrongly dated to the Byzantine or early medieval period.
The research was carried out by archaeologists who believe studying photographs taken during covert surveillance may help uncover and identify thousands of archaeological sites around the world.
The recently declassified covert US Corona satellite intelligence programme of the 1960s-80s includes around 900,000 photographs from around the world.
Bill Hanson, professor of Roman archaeology at Glasgow University, said: "We have enough evidence here to demonstrate the existence of a chronologically complex Roman frontier system, and the most easterly example of a man-made barrier in the Roman Empire. It is an incredibly important discovery."
Ioana Oltean, a senior lecturer in archaeology at Exeter University, said: "Photographs from military surveillance are proving to be of enormous benefit in showing us our lost archaeological heritage."