The Peace of Westphalia (German: Westfälischer Friede, pronounced [vɛstˈfɛːlɪʃɐ ˈfʁiːdə] (listen)) is the collective name for two peace treaties signed in October 1648 in the Westphalian cities of Osnabrück and Münster. They ended the Thirty Years' War (1618–1648) and Eighty Years' War (1568–1648), and brought peace to the Holy Roman Empire, closing a calamitous period of European history that killed approximately eight million people. The Holy Roman Emperor (Ferdinand III of Habsburg), the Spanish Monarchy, the kingdoms of France and Sweden, the United Provinces (Netherlands), and their respective allies among the princes of the Holy Roman Empire participated in these treaties.The negotiation process was lengthy and complex. Talks took place in two cities, because each side wanted to meet on territory under its own control. A total of 109 delegations arrived to represent the belligerent states, but not all delegations were present at the same time. Two treaties were signed to end the war in the Empire: the Treaty of Münster and the Treaty of Osnabrück. These treaties ended the Thirty Years' War in the Holy Roman Empire, with the Habsburgs (rulers of Austria and Spain) and their Catholic allies on one side, battling the Protestant powers (Sweden and certain Holy Roman principalities) allied with France, which was Catholic but strongly anti-Habsburg under King Louis XIV. The separate Peace of Münster ended the Eighty Years' War between Spain and the United Provinces.

Joachim Whaley, a leading English-language historian of the Holy Roman Empire, mentions that later commentators such as Leibniz, Rousseau, Kant, and Schiller eulogized the Peace of Westphalia as the first step towards a universal peace, but he points out that "their projections for the future should not be mistaken for descriptions of reality".Scholars of international relations have identified the Peace of Westphalia as the origin of principles crucial to modern international relations, collectively known as Westphalian sovereignty, though other historians argue that this is largely a myth invented after the fact.