Cardinal Jules Mazarin (, also UK: , US: , French: [ʒyl mazaʁɛ̃]; 14 July 1602 – 9 March 1661), born Giulio Raimondo Mazzarino, (Italian: [ˈdʒuːljo raiˈmondo maddzaˈriːno]) or Mazarini, was an Italian cardinal, diplomat and politician who served as the chief minister to the Kings of France Louis XIII and Louis XIV from 1642 to his death. In 1654, he acquired the title Duke of Mayenne and in 1659 that of 1st Duke of Rethel and Nevers.

After serving as a papal diplomat for Pope Urban VIII, Mazarin offered his diplomatic services to Cardinal Richelieu and moved to Paris in 1640. After the death of Richelieu in 1642, Mazarin took his place as first minister and then of Louis XIII in 1643. Mazarin acted as the head of the government for Anne of Austria, the regent for the young Louis XIV. Mazarin was also made responsible for the king's education until he came of age.

The first years of Mazarin in office were marked by military victories in the Thirty Years' War, which he used to make France the main European power and establish the Peace of Westphalia (1646–1648). A major uprising against Anne of Austria and Mazarin, called the Fronde and led by the nobles of the Parliament of Paris, broke out in Paris in 1648, followed by a second Fronde, led by Louis, Grand Condé, who had turned from his chief ally to his chief enemy. Mazarin took Anne of Austria and Louis XIV out of Paris and then shifted his base to Germany for a time. Turenne, a general loyal to Louis XIV and Mazarin, defeated Condé, and Mazarin made a triumphal return to Paris in 1653.

The last years of Mazarin's life, between 1657 and his death in 1661, were marked by a series of major diplomatic victories. In 1657 he made a military alliance with England. In 1658 he unveiled the League of the Rhine, a new group of 50 small German principalities, which were now linked by a treaty with France. The same year, Marshal Turenne decisively defeated the army of Condé at the Battle of the Dunes in Flanders. Between February and June 1659, Mazarin conducted intensive negotiations with the Spanish. On 7 November 1659, Spain signed the Treaty of the Pyrenees, which added Artois, the Cerdagne and Roussillon as new provinces of France. That was followed in June 1660 by an even more important diplomatic event that had been carefully arranged by Mazarin, the marriage of Louis XIV with Maria Theresa of Spain. The marriage took place in Saint-Jean-de-Luz. The couple made a triumphant entry into Paris on 26 August 1660. The marriage and accompanying agreements ended, at least for a time, the long and costly wars between the Habsburgs and France. Exhausted by his diplomatic efforts, Mazarin died in 1661.

Mazarin, as the de facto ruler of France for nearly two decades, played a crucial role in establishing the Westphalian principles that would guide European states' foreign policy and the prevailing world order. Some of the principles, such as the nation state's sovereignty over its territory and domestic affairs and the legal equality among states, have remained the basis of international law to this day.

In addition to his diplomacy, Mazarin was an important patron of the arts. He introduced Italian opera on a grand scale to Paris and assembled a remarkable art collection, much of which today can be seen in the Louvre. He also founded the Bibliothèque Mazarine, the first true public library in France, which is now found in the Institut de France, across the Seine from the Louvre.