The Charter of the United Nations (UN) is the foundational treaty of the United Nations, an intergovernmental organization. It establishes the purposes, governing structure, and overall framework of the UN system, including its six principal organs: the Secretariat, the General Assembly, the Security Council, the Economic and Social Council, the International Court of Justice, and the Trusteeship Council.

The UN Charter mandates the UN and its member states to maintain international peace and security, uphold international law, achieve "higher standards of living" for their citizens, address "economic, social, health, and related problems", and promote "universal respect for, and observance of, human rights and fundamental freedoms for all without distinction as to race, sex, language, or religion". As a charter and constituent treaty, its rules and obligations are binding on all members and supersede those of other treaties.

During the Second World War, the Allies—formally known as the United Nations—agreed to establish a new postwar international organization. Pursuant to this goal, the UN Charter was discussed, prepared, and drafted during the San Francisco Conference that began 25 April 1945, which involved most of the world's sovereign nations. Following two-thirds approval of each part, the final text was unanimously adopted by delegates and opened for signature on 26 June 1945; it was signed in San Francisco, United States, by 50 of the 51 original member countries.

The Charter entered into force on 24 October 1945, following ratification by the five permanent members of the United Nations Security Council—China, France, the Soviet Union, the United Kingdom, and the United States—and a majority of the other signatories; this is considered the official starting date of the United Nations, with the first session of the General Assembly, representing all 51 initial members, opening in London the following January. The General Assembly formally recognized 24 October as United Nations Day in 1947, and declared it an official international holiday in 1971. With 193 parties, most countries have now ratified the Charter.