Mexico City is the capital and largest city of Mexico, as well as the most populous city in North America. It is one of the most important cultural and financial centers in the world, and is classified as an Alpha world city according to the Globalization and World Cities Research Network (GaWC) 2024 ranking. Mexico City is located in the Valley of Mexico within the high Mexican central plateau, at an altitude of 2,240 meters (7,350 ft). The city has 16 boroughs or demarcaciones territoriales, which are in turn divided into neighborhoods or colonias.

The 2020 population for the city proper was 9,209,944, with a land area of 1,495 square kilometers (577 sq mi), making it the second largest Spanish-speaking city proper in the world. According to the most recent definition agreed upon by the federal and state governments, the population of Greater Mexico City is 21,804,515, which makes it the world's sixth-largest metropolitan area and the second-largest urban agglomeration in the Western Hemisphere (behind São Paulo, Brazil). Greater Mexico City has a GDP of $411 billion in 2011, which makes it one of the most productive urban areas in the world. The city was responsible for generating 15.8% of Mexico's GDP, and the metropolitan area accounted for about 22% of the country's GDP. If it were an independent country in 2013, Mexico City would be the fifth-largest economy in Latin America.

Mexico City is the oldest capital city in the Americas and one of two founded by Indigenous people. The city was originally built on a group of islands in Lake Texcoco by the Mexica around 1325, under the name Tenochtitlan. It was almost completely destroyed in the 1521 siege of Tenochtitlan and subsequently redesigned and rebuilt in accordance with the Spanish urban standards. In 1524, the municipality of Mexico City was established, known as México Tenochtitlán, and as of 1585, it was officially known as Ciudad de México (Mexico City). Mexico City played a major role in the Spanish colonial empire as a political, administrative, and financial center. Following independence from Spain, the region around and containing the city was established as the new and only Mexican federal district (Spanish: Distrito Federal or DF) in 1824.

After years of demanding greater political autonomy, in 1997 residents were finally given the right to elect both a head of government and the representatives of the unicameral Legislative Assembly by election. Ever since, left-wing parties (first the Party of the Democratic Revolution and later the National Regeneration Movement) have controlled both of them. The city has several progressive policies, such as elective abortions, a limited form of euthanasia, no-fault divorce, same-sex marriage, and legal gender change. On 29 January 2016, it ceased to be the Federal District (DF) and is now officially known as Ciudad de México (CDMX). These 2016 reforms gave the city a greater degree of autonomy and made changes to its governance and political power structures. A clause in the Constitution of Mexico, however, prevents it from becoming a state within the Mexican federation, as long as it remains the capital of the country.

Up until 2013, it was common to refer to the city by the initialism "DF" from Distrito Federal de México. Since 2013, use of the abbreviation "CDMX" (Ciudad de México) has been more common, particularly by government.

The city is colloquially known as Chilangolandia after the locals' nickname chilangos. Chilango is used pejoratively by people living outside Mexico City to "connote a loud, arrogant, ill-mannered, loutish person". For their part those living in Mexico City designate insultingly those who live elsewhere as living in la provincia ('the provinces', 'the periphery') and many proudly embrace the term chilango. Residents of Mexico City are formally called capitalinos (in reference to the city being the capital of the country), but "[p]erhaps because capitalino is the more polite, specific, and correct word, it is almost never utilized".

Mexico City was traditionally known as La Ciudad de los Palacios ("the City of the Palaces"), a nickname attributed to Baron Alexander von Humboldt when visiting the city in the 19th century, who, sending a letter back to Germany, said Mexico City could rival any major city in Europe. But it was English politician Charles Latrobe who really penned the following: "... look at their works: the moles, aqueducts, churches, roads—and the luxurious City of Palaces which has risen from the clay-built ruins of Tenochtitlan...", on page 84 of the Letter V of The Rambler in Mexico.

During the colonial period, the city's motto was "Muy Noble e Insigne, Muy Leal e Imperial" (Very Noble and Distinguished, Very Loyal and Imperial). During Andrés Manuel López Obrador's administration (2018–24) a political slogan was introduced: la Ciudad de la Esperanza (lit. 'The City of Hope'). This motto was quickly adopted as a city nickname but has faded since the new motto, Capital en Movimiento ("Capital in Movement"), was adopted by the administration headed by Marcelo Ebrard; this latter motto is not really treated as a nickname.