Mexico City (Spanish: Ciudad de México) is the capital and largest city in the country of Mexico. Mexico City is also the Federal District (Distrito Federal), seat of the federal government. The Federal District is considered a federal entity within Mexico and is not part of any one of the 31 Mexican states but belongs to the federation as a whole. It is the most important political, cultural, and financial center in the country. Mexico City is also considered an important financial center in Latin America. Mexico City is also considered a global city. Located in the Valley of Mexico, a large valley in the high plateaus at the center of Mexico, at an altitude of 2,240 metres (7,350 ft), the city consists of sixteen boroughs.

The 2009 estimated population for the city proper exceeds 8.84 million people, and with a land area of 1,485 square kilometres (573 sq mi), Mexico City is one of the most densely populated cities in the country. According to the most recent definition agreed upon by the federal and state governments, the Mexico City metropolitan area population is 21.2 million people, making it the largest metropolitan area in the Americas and the third largest agglomeration in the world. The city had a Gross Domestic Product (GDP) of $390 billion USD in 2008, making Mexico City the eighth richest city in the world. If Mexico City were an independent nation, its GDP would still be the 30th largest economy in the world. The city was responsible for generating 21% of Mexico's Gross Domestic Product and the metropolitan area accounted for 34% of total national GDP. According to estimates, as of 2008, the city proper, as opposed to the metropolitan area, had a nominal income per capita of $25,258 USD, above the national average, and on par with the GDP per capita of Portugal, which has a comparable population, and significantly above nations such as South Korea, and Czech Republic.

The city was originally built on an island of Lake Texcoco by the Aztecs in 1325 as Tenochtitlan, which was almost completely destroyed in the siege of 1521, and subsequently redesigned and rebuilt in accordance with the Spanish urban standards. In 1524, the municipality of Mexico City was established, known as México Tenustitlán, and as of 1585 it was officially known as La Ciudad de México (Mexico City). Mexico City served as the political, administrative and financial centre of a major part of the Spanish colonial empire. After independence from Spain was achieved, the Federal District was created in 1824.

After years of demanding greater political autonomy, residents were given the right to directly elect the Head of Government and the representatives of the unicameral Legislative Assembly by popular vote in 1997. Ever since, the left-wing Party of the Democratic Revolution (PRD) has controlled both of them. In recent years, the local government has passed a wave of liberal policies, such as abortion on request to any woman up to 12 weeks into a pregnancy, a limited form of euthanasia, and LGBT rights. The city became the first jurisdiction in Latin America to legalize same-sex marriage.

Urbanization has had a serious negative effect on the city's environment. Pollution, dwindling water supply and poor air quality have been some of the city's largest problems improved in recent years by regulating the circulation and renovating the vehicle park. Many factors such as industrial growth and a demographic boom (from 3 million in 1950 to some 20 million in the metropolitan area today) have contributed to this situation. More than 3.5 million vehicles are now in the city streets.