Rio de Janeiro , or simply Rio, is the second most populous municipality in Brazil and the sixth largest city in the Americas. The metropolis is anchor to the Rio de Janeiro metropolitan area, the second most populous metropolitan area in Brazil and seventh most populous in the Americas. Rio de Janeiro is the capital of the state of Rio de Janeiro, Brazil's third most populous state. Part of the city has been designated as a World Heritage Site, named "Rio de Janeiro: Carioca Landscapes between the Mountain and the Sea", by UNESCO on 1 July 2012 as a Cultural Landscape.

Founded in 1565 by the Portuguese, the city was initially the seat of the Captaincy of Rio de Janeiro, a domain of the Portuguese Empire. Later, in 1763, it became the capital of the State of Brazil, a state of the Portuguese Empire. In 1808, when the Portuguese Royal Court transferred itself from Portugal to Brazil, Rio de Janeiro became the chosen seat of the court of Queen Maria I of Portugal, who subsequently, in 1815, under the leadership of her son, the Prince Regent, and future King João VI of Portugal, raised Brazil to the dignity of a kingdom, within the United Kingdom of Portugal, Brazil, and Algarves. Rio stayed the capital of the pluricontinental Lusitanian monarchy until 1822, when the War of Brazilian Independence began. This is one of the few instances in history that the capital of a colonising country officially shifted to a city in one of its colonies. Rio de Janeiro subsequently served as the capital of the independent monarchy, the Empire of Brazil, until 1889, and then the capital of a republican Brazil until 1960 when the capital was transferred to Brasília.

Rio de Janeiro has the second largest municipal GDP in the country,  and 30th largest in the world in 2008,  estimated at about R$343 billion (IBGE, 2008) (nearly US$201 billion). It is headquarters to Brazilian oil, mining, and telecommunications companies, including two of the country's major corporations—Petrobras and Vale—and Latin America's largest telemedia conglomerate, Grupo Globo. The home of many universities and institutes, it is the second-largest center of research and development in Brazil, accounting for 17% of national scientific output according to 2005 data.

Rio de Janeiro is one of the most visited cities in the Southern Hemisphere and is known for its natural settings, Carnival, samba, bossa nova, and balneario beaches such as Barra da Tijuca, Copacabana, Ipanema, and Leblon. In addition to the beaches, some of the most famous landmarks include the giant statue of Christ the Redeemer atop Corcovado mountain, named one of the New Seven Wonders of the World; Sugarloaf Mountain with its cable car; the Sambódromo (Sambadrome), a permanent grandstand-lined parade avenue which is used during Carnival; and Maracanã Stadium, one of the world's largest football stadiums.

Rio de Janeiro will host the 2016 Summer Olympics and the 2016 Summer Paralympics—the first time a South American and Portuguese-speaking country will host these events, and the third time the Olympics will be held in a Southern Hemisphere city.  The Maracanã Stadium held the finals of the 1950 and 2014 FIFA World Cups, the 2013 FIFA Confederations Cup, and the XV Pan American Games including its opening and closing ceremonies. Rio de Janeiro also hosted World Youth Day in 2013.