Guy Fawkes was caught and executed
The explosives are in remarkably good condition according to officials at the British Library, where it was discovered in the basement.
It came to light in an old stationery box - with a photograph of the Houses of Parliament on the top - which was given to the library in 1995 as part of the John Evelyn Collection.
Evelyn was a wealthy seventeenth-century diarist whose family had made its money in gunpowder manufacture.
Handwritten message
His papers were given to the Library in 1995 and curators found the gunpowder as they were cataloguing the collection.
The box contains a solid bar of gunpowder and some more in granular form, wrapped in twists of paper.
Also in the box is a note in John Evelyn's handwriting suggesting the gunpowder belonged to Guy Fawkes, and a 19th century envelope with a handwritten message to the same effect.
Unfortunately the envelope also carries another sentence, dated 1952, which says: "But there was none left!", leaving open the question of whether the British Library's gunpowder really was owned by Guy Fawkes.
Oldest surviving sample
Fawkes and his fellow Roman Catholic conspirators tried to blow up parliament during the state opening by James I on 5 November 1605.
The plot was foiled and the conspirators executed.
The British Library says it does not have an explosives licence, and will be handing the gunpowder over to the Royal Armouries.
It could be the oldest surviving gunpowder sample.