Ercole Antonio Mattioli (1 December 1640 – 1694) was an Italian politician, who was a minister of Duke Charles IV of Mantua.
He worked as a minister of Charles IV, Duke of Mantua, who, as Marquess of Montferrat, possessed the strategic fortress of Casale Monferrato.
Mattioli successfully negotiated the trade and in gratitude, Louis XIV rewarded him lavishly.
However, when the French were about to occupy the castle, Mattioli revealed the secret deal to the governments of Austria, Savoy, Spain and Venice, possibly for additional rewards.
Another prisoner at Pignerol was Eustache Dauger, the so-called Man in the Iron Mask, which some have said was actually made of black velvet cloth.