Philippe de Mornay (5 November 1549 – 11 November 1623), seigneur du Plessis Marly, usually known as Du-Plessis-Mornay or Mornay Du Plessis, was a French Protestant writer and member of the anti-monarchist Monarchomaques.
He was born in Buhy, now situated in Val-d'Oise. His mother had leanings toward Protestantism, but his father tried to counteract her influence by sending him to the Collège de Lisieux at Paris. On his father's death in 1559, however, the family formally adopted the reformed faith. Mornay studied law and jurisprudence at the University of Heidelberg in 1565 and the following year Hebrew and German at the University of Padua. During the French Wars of Religion in 1567, he joined the army of Louis I de Bourbon, prince de Condé, but a fall from his horse prevented him from taking an active part in the campaign. His career as Huguenot apologist began in 1571 with the work Dissertation sur l'Église visible, and, as a diplomat in 1572, he undertook a confidential mission for Admiral de Coligny to William the Silent, Prince of Orange.
He escaped the St. Bartholomew's Day Massacre by the aid of a Catholic friend, taking refuge in England. Returning to France towards the end of 1573, he participated during the next two years with various success in the campaigns of the future Henry IV of France, then only King of Navarre. He was taken prisoner by the Duke of Guise on 10 October 1575 was but ransomed for a small sum, which was paid by Charlotte Arbaleste, whom he married shortly afterwards at Sedan.