There are only 24 known editions of the document, considered one of the most important in history in the world.
It's thought it could be worth up to £10 million pounds.
Speaking to the Mailonline from Paris, Professor Nicholas Vincent, of the University of East Anglia, who authenticated the document, said: "It is a fantastic discovery which comes in the week that the four other known versions were brought together at the Houses of Parliament.
"It is a fantastic piece of news for Sandwich which puts it in a small category of towns and institutions that own a 1300 issue."
It is understood that Sandwich does not intend to sell its Magna Carta but instead is hoping to benefit from its potential as a tourist attraction.
Paul Graeme, mayor of Sandwich Town Council, said: "On behalf of Sandwich Town Council, I would like to say that we are absolutely delighted to discover that an original Magna Carta and original Charter of the Forest, previously unknown, are in our ownership.
To own one of these documents, let alone both, is an immense privilege given their international importance.
'Perhaps it is fitting that they belong to a town where Thomas Paine lived, who proposed in his pamphlet Common Sense a Continental Charter for what were then the American colonies, 'answering to what is called the Magna Carta of England ... securing freedom and property to all men, and ... the free exercise of religion'.
"Through the American Declaration of Independence, continuing in the Universal Declaration of Human Rights, Magna Carta still underpins individual liberties worldwide.
To own such a document , and the Charter of the Forest is an honour and a great responsibility, " he added.