The circular structure was built by Edward III in the 14th century to house the round table intended to seat the original 300 Knights of the Garter.
Archaeological proof of the 61-metre-diameter building was uncovered by members of Channel 4's Time Team in the castle's quadrangle.
Although the stones have been removed, ghost walls - rubble in-fill where they were originally located - remained in place.
Presenter Tony Robinson said the discovery could help settle years of debate among historians over the existence of the building.
"The round table building is one of our most significant ever archaeological finds,” Robinson said.
"It is something that helped to establish Arthurian legends of the knights of the round table.
"We set out to uncover the walls of the building, and they are just where we hoped.
"Experts have speculated about the structure for centuries, but they have never been able to find the actual building."
More relics uncovered
It was one of several remarkable finds made by archaeologists given unprecedented access to three royal residences.
Huge teams of experts moved into Buckingham Palace, Windsor Castle and The Palace of Holyroodhouse, in Edinburgh, for a mammoth three-day dig.
They brought in equipment from excavators to toothbrushes and centuries-old records to magnetic gradiometer surveying machines to help locate hidden treasures.
Artists and graphic artists were brought in to create impressions of how previous generations may have viewed the buildings and grounds.
At Buckingham Palace, finds ranged from a small piece of pre-Roman flint to a gold earring or piece of necklace, possibly from the Victorian era.
Experts also uncovered a glazed stoneware beer mug that was probably thrown away by workmen landscaping the gardens in about 1700.
Other artefacts included tobacco pipes dating back to the mid 17th century, one with the words "Westminster" and "Brown" written on it, an iron hoe and even a 19th century toy.
At Holyroodhouse, the Queen's official Scottish residence, a 17th or 18th century seal, probably used to stamp wax on documents, was uncovered.
Presenter Neil Oliver said experts had also unearthed some of the foundations of an ancient 12th century monastery.
The digs were allowed to go ahead after months of negotiations with representatives of the royal family.
A spokesman for Buckingham Palace said all the findings will be catalogued and the survey results will be added to the royal archive at Windsor.