The 4,500-year-old stones, some measuring 15ft, were discovered under 3ft of earth using ground-penetrating radar at Durrington Walls “superhenge”.
The monument was on “an extraordinary scale” and unique, researchers said.
The Stonehenge Hidden Landscapes team has been creating an underground map of the area in a five-year project.
The monument is just under two miles (3km) from Stonehenge, Wiltshire, and is thought to have been a ritual site.
The stones are believed to have been deliberately toppled over the south-eastern edge of the bank of the circular enclosure before being incorporated into it.
Lead researcher Vince Gaffney, of the University of Bradford, said: “We don’t think there’s anything quite like this anywhere else in the world.
“This is completely new and the scale is extraordinary.”
Archaeologist Nick Snashall said: “The presence of what appear to be stones, surrounding the site of one of the largest neolithic settlements in Europe adds a whole new chapter to the Stonehenge story.”