The University of Sheffield's archaeological consultancy firm ARCUS made the unexpected find in Sheffield's city centre.
The discovery was made during the team's excavation at Carmel House on Fargate in Sheffield city centre, as part of the redevel
opment of the site by Hermes Property Unit Trust.
University academics said it was "unprecedented evidence" about how Sheffield would have looked in medieval times, when it was a small market town until its massive growth during the industrial revolution.
The dating of the well, dug into sandstone bedrock, suggested it was contemporary with the rebuilding of Sheffield Castle in stone in 1270 and the granting of Sheffield's market charter by Edward I in 1296.
Important pottery finds were also recovered, further suggesting that the well was in use by 1300, and had been filled in during the 17th century around the time of the Civil War.
The medieval ceramics included jugs made in the Hallgate area of Doncaster and other vessels from the Humber estuary.
Animal bones, plant remains and microscopic pollen grains were also found in the waterlogged conditions of the well.
Analysis of the finds will now take place in the laboratories of the university's archaeology department.
University archaeologist Steve Baker said: "It's fantastic to find this evidence which has lain undisturbed beneath the town for eight hundred years. This represents a goldmine of information relating to medieval Sheffield."
Director of ARCUS, Jim Symonds, added: "Sheffield is world famous for its achievements during the industrial age. However, this important discovery reminds us that Sheffield was also a thriving medieval town."
Two years ago city planners passed controversial plans to turn the Grade II listed Carmel House, which stands on the corner of Fargate and Norfolk Row in the city centre, into shops.
This was to make way for a £10m redevelopment of the Victorian building.
The move had raised concerns from conservationists, including English Heritage, but councillors agreed to the plans on the grounds the front facade would be retained
Work began earlier this year.
