The document was discovered in the West Sussex Record Office in Chichester, where staff had been unaware of its significance.
The original Declaration proclaimed the first 13 colonies free from British rule and was signed in Philadelphia on 4 July, 1776 - now America's Independence Day.
Harvard University researchers Emily Sneff and Danielle Allen made the discovery while compiling records for a database.
It began in August 2015, when they stumbled on a simple one-line entry in the office's catalogue: "Manuscript copy, on parchment, of the Declaration in Congress of the thirteen United States of America."
They thought it would probably be a 19th century reproduction - but it turned out to be a "big mystery" and far more significant than they imagined.
Now christened the 'Sussex Declaration', it measures the same size as the original, 24in by 30in, but has been written horizontally.
