Melchior de Polignac (October 11, 1661 – November 20, 1742) was a French diplomat, Roman Catholic cardinal and neo-Latin poet.
Second son of Armand XVI, marquis de Polignac and Marquis Chalancon, Governor of Puy; and Jacqueline de Beauvoir -Grimoard-de Roure (his third wife), Melchior de Polignac was born at Chateau de la Ronte, near Puy en Vélay, Lavoûte-sur-Loire, Haute-Loire, Auvergne.
A precocious child, he was taken by his uncle to Paris, and installed in the Jesuit Collège de Clermont (later named the Collège de Louis le Grand). At the appropriate time, he passed to the Collège de Harcourt, where thanks to the misdirected efforts of a teacher who was an enthusiast for Aristotle, Polignac adopted the opposite view and became a Cartesian. His thesis in Theology at the Sorbonne (1683) discussed the Kings of Judah who had destroyed the "high places". He was either prescient, or aware of discussion around Louis XIV which led in two years to the revocation of the Edict of Nantes (1685) and the removal of the Huguenot "high places". He had somehow attracted the patronage of the Cardinal Emmanuel de la Tour d'Auvergne de Bouillon, who took Polignac with him when he went to Rome for the Conclave following the death of Pope Innocent XI on August 12, 1689. Bouillon chose Polignac as one of his Conclavists. When the new Pope, Alexander VIII (Ottoboni) was elected, he assisted Cardinal de Bouillon and the French Ambassador, the Duc de Chaulnes, in attempting to improve relations between Louis XIV and the Holy See. Polignac was sent back to France to report to Louis, who immediately sent him back to Rome with further instructions. He was still in Rome when Alexander VIII died after less than sixteen months on the throne, and was again Conclavist of Cardinal de Bouillon in the Conclave of 1691 that elected Innocent XII (Pignatelli).