The marching camp - complete with 26 fire pits - was found in Ayr as building of the town's new academy got underway.
The Roman-era relevance of the site was at first not obvious given only fragments of much earlier Neolithic Pottery and an Iron Age bangle were discovered.
Until now, the only two known routes for the Roman invasion of southern Scotland were further to the east, with today's M74 and A68 roads following the same course.
The new marching camp at Ayr reveals another route down the west coast of Scotland towards the south-west tip of the country, from where Ireland is readily visible.
Iraia Arabaolaza, who directed the excavation for GUARD Archaeology, said: "The Roman features comprised 26 large, often double, fire-pits that were distributed evenly in two parallel rows 30m apart.
Burnt clay fragments were also recovered as well as a series of ash pits, which were filled with burn and charcoal-rich soil made from the raked-out contents of the clay-domed ovens.
Radiocarbon testing on the fire pits dated them to between 77 and 86AD to 90AD, which chimes with the invasion of Scotland by the Roman General Agricola.
Ms Arabaolaza said the Ayr marching camp is 20 miles south from the nearest known camp at Girvan, the equivalent of a day's march for a Roman soldier.
Another camp can be found to the north-east near Strathaven.
"Altogether, this suggests that the site was chosen as a strategic location for the Roman conquest of Ayrshire," Ms Arabaolaza said.
Roman marching camps were the temporary bases of a tented army on campaign.
Mr Arabaolaza said dimension of the area given to the fire pits suggested the camp may have been home to around 640 legionaries.
It is also possibly that only a portion of the camp has been found, she added.
The excavation of the Ayr Academy site also revealed a raft of earlier archaeological remains.
Evidence of Bronze Age ritual activity from the late third and second millennium BC, a Neolithic settlement from the fourth millennium BC and a Mesolithic hunter/gatherer camp from the sixth millennium BC was also discovered.
Ms Arabaolaza said the site one of the earliest and most complex prehistoric sites in this part of the west coast.
She added: "To put this into perspective, the earliest occupation of the Ayr Academy site goes back to around 5,200BC, roughly two-and-a-half times as old as the Roman Marching Camp is to us.
"As the excavation at Ayr Academy demonstrates, Scotland was not an untouched wild landscape that the Romans marched into in AD79 but already an ancient land inhabited by communities whose culture and heritage stretch back millennia."
Ms Arabaolaza presented her findings to the Archaeological Research in Progress 2019 Conference, organised by the Society of Antiquaries of Scotland, at the weekend.