The United States Ambassador to the United Nations is the leader of the U.S. delegation, the U.S. Mission to the United Nations. The position is more formally known as the "Permanent Representative of the United States of America to the United Nations, with the rank and status of Ambassador Extraordinary and Plenipotentiary, and Representative of the United States of America in the Security Council of the United Nations"; it is also known as the U.S. Permanent Representative, or "Perm Rep", to the United Nations.

The U.S. Permanent Representative, currently Susan Rice, is charged with representing the United States on the U.N. Security Council and during almost all plenary meetings of the General Assembly, except in the rare situation in which a more senior officer of the United States (such as the U.S. Secretary of State or the President of the United States) is present. Like all United States ambassadors, he or she must be nominated by the U.S. President and confirmed by the Senate.

Many prominent U.S. politicians and diplomats have held the post, including Henry Cabot Lodge, Jr., Adlai Stevenson, George H. W. Bush, Daniel Patrick Moynihan, Andrew Young, Dr. Jeane Kirkpatrick, Richard Holbrooke, Dr. Madeleine Albright, Bill Richardson, and John Danforth.

The United States of America (commonly abbreviated to the United States, the U.S., the USA, America, and the States) is a federal constitutional republic comprising fifty states and a federal district. The country is situated mostly in central North America, where its forty-eight contiguous states and Washington, D.C., the capital district, lie between the Pacific and Atlantic Oceans, bordered by Canada to the north and Mexico to the south. The state of Alaska is in the northwest of the continent, with Canada to the east and Russia to the west, across the Bering Strait. The state of Hawaii is an archipelago in the mid-Pacific. The country also possesses several territories in the Pacific and Caribbean.

At 3.79 million square miles (9.83 million km²) and with over 312 million people, the United States is the third or fourth largest country by total area, and the third largest by both land area and population. It is one of the world's most ethnically diverse and multicultural nations, the product of large-scale immigration from many countries. The U.S. economy is the world's largest national economy, with an estimated 2011 GDP of $15.1 trillion (22% of nominal global GDP and over 19% of global GDP at purchasing-power parity). Per capita income is the world's sixth-highest.

A United Nations Permanent Representative (sometimes called a "UN ambassador") is the head of a diplomatic mission to the United Nations Organization (or United Nations, UN). Of these, the most high-profile UN Permanent Representatives are those assigned to headquarters in New York City (U.S.). However, member states also appoint Permanent Representatives to the other UN offices in Geneva (Switzerland), Vienna (Austria), and Nairobi (Kenya).

Many countries, including the United States, call their UN Permanent Representatives "U.N. ambassadors". Although a Permanent Representative holds the equivalent diplomatic rank of an ambassador (or Chief of Mission or High Commissioner), he or she is accredited to an international organisation, and not to a Head of State (as a nation's ambassador would be) or to a Head of Government (as a High Commissioner would be).

Some diplomats are representatives to UN councils, such as the Economic and Social Council of the United Nations.

UNESCO has Permanent Delegates heading the diplomatic missions to the organisation, rather than Permanent Representatives. However, there are also UNESCO Goodwill Ambassadors, such as many American celebrities who act as a UNESCO Goodwill Ambassador to a particular nation. UNHCR has similar UNHCR Goodwill Ambassadors.

The United Nations (abbreviated UN in English, and ONU in its other official languages), is an international organization whose stated aims are facilitating cooperation in international law, international security, economic development, social progress, human rights, and achievement of world peace. The UN was founded in 1945 after World War II to replace the League of Nations, to stop wars between countries, and to provide a platform for dialogue. It contains multiple subsidiary organizations to carry out its missions.

There are 193 member states, including every internationally recognised sovereign state in the world but Vatican City. From its offices around the world, the UN and its specialized agencies decide on substantive and administrative issues in regular meetings held throughout the year. The organization has six principal organs: the General Assembly (the main deliberative assembly); the Security Council (for deciding certain resolutions for peace and security); the Economic and Social Council (for assisting in promoting international economic and social cooperation and development); the Secretariat (for providing studies, information, and facilities needed by the UN); the International Court of Justice (the primary judicial organ); and the United Nations Trusteeship Council (which is currently inactive). Other prominent UN System agencies include the World Health Organization (WHO), the World Food Programme (WFP) and United Nations Children's Fund (UNICEF). The UN's most prominent position is Secretary-General which has been held by Ban Ki-moon of South Korea since 2007.

This is a list of ambassadors of the United States to individual nations of the world, to international organizations, to past nations, and ambassadors-at-large.

Ambassadors are nominated by the President and confirmed by the Senate. An ambassador can be appointed during a recess of the Senate, but can serve only to the end of the next session of Congress unless subsequently confirmed by the Senate. Ambassadors serve "at the pleasure of the President," which means that they can be dismissed at any time.

An ambassador may be a career foreign service officer or a political appointee. In most cases, U.S. ambassadors who are career foreign service officers serve a tour of approximately three years in a foreign post. Ambassadors who are political appointees will customarily tender their resignations upon inauguration of a new President. As embassies fall within the Department of State, ambassadors answer to the Secretary of State.

Current ambassadors from the United States to international organizations