The fort was found by instruments which can detect burried walls
Experts describe the discovery of a fort as filling in the "missing link" in Cumbria's Roman occupation.
A dig at Castlerigg, near Keswick, has uncovered a camp which dates back to the first Century.
It was discovered by chance during a search for a second stone circle or 14th Century castle.
The underground remains, which are the size of eight football pitches, back up theories of a Roman presence in the Keswick area.
The dig was led by Bassenthwaite Reflections, with the permission of English Heritage.
Aerial photographs and ground searches had revealed markings which were originally thought to be the remains of a manor house.
Much of the work was done by volunteers
However, magnetometers, instruments which can detect buried walls, showed the large enclosed structure.
Archaeologist Mark Graham said he thought there was little doubt that the find was a temporary camp, capable of holding large numbers of troops.
"It could have been an important part of the first push to 'Romanise' the area, perhaps as early as 70 AD, a militarisation that extended across the county for 300 years," he said.
"It possibly serviced campaigns into Scotland and acted as a base for soldiers heading north, or withdrawing."
He added that there was nothing to see above the ground, but it was clear to see why the site was chosen.
"In sight of Castlerigg Stone Circle, which was already 3,000 years old at the time of the Roman occupation, the elevated position was strategically well placed for defence," Mr Graham said.
"It also has lovely views over Bassenthwaite and to other Roman camps at Troutbeck."
No formal excavation of the site is planned but its hoped further work will uncover important artefacts.