Gaston, Duke of Orléans (24 April 1608 – 2 February 1660), was the third son of King Henry IV of France and his wife Marie de Medici. As a son of the king, he was born a Fils de France. He later acquired the title Duke of Orléans, by which he was generally known during his adulthood. As the eldest surviving brother of King Louis XIII, he was known at court by the traditional honorific Monsieur.

Gaston Jean Baptiste was born at the Palace of Fontainebleau on 24 April 1608 and at birth was given the title of Duke of Anjou. In 1626, at the time of his marriage to the young Marie de Bourbon, Duchess of Montpensier, he received in appanage (with their respective titles) the duchies of Orléans and Chartres, and the county of Blois. He had nominal command of the army which besieged La Rochelle in 1628, having already entered upon a course of political intrigue that would occupy the remainder of his life. He was the heir presumptive to the throne of France from the death of his brother Nicolas Henri in 1611 until the birth of his elder brother's first son in 1638.

On two occasions, he had to leave France for conspiring against the government of his mother and her Prime Minister Cardinal Richelieu. After waging an unsuccessful war in Languedoc leading to the Battle of Castelnaudary in 1632, he took refuge in Flanders. Reconciled with his brother Louis XIII, he plotted against Richelieu in 1635, fled from the country again, and then submitted to the King and the Cardinal.

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