Rio de Janeiro (Portuguese: [ˈʁi.u d(ʒi) ʒɐˈne(j)ɾu] (listen); literally 'River of January'), or simply Rio, is the capital of the state of the same name, Brazil's third most populous state, and the second most populous city in Brazil, after São Paulo. Listed by the GaWC as a beta global city, Rio de Janeiro is the sixth most populous city in the Americas. Part of the city has been designated as a World Heritage Site, named "Rio de Janeiro: Carioca Landscapes between the Mountain and the Sea", 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. In 1763, it became the capital of the State of Brazil, a state of the Portuguese Empire. In 1808, when the Portuguese Royal Court moved to Brazil, Rio de Janeiro became the seat of the court of Queen Maria I of Portugal. She subsequently, under the leadership of her son the prince regent João VI of Portugal, raised Brazil to the dignity of a kingdom, within the United Kingdom of Portugal, Brazil, and Algarves. Rio remained as the capital of the pluricontinental Lusitanian monarchy until 1822, when the Brazilian War of Independence began. This is one of the few instances in history that the capital of a colonizing 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. This is estimated at R$343 billion. In the city are the headquarters of 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 percent of national scientific output according to 2005 data. Despite the high perception of crime, the city actually has a lower incidence of crime than most state capitals in Brazil.

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 was the host of the 2016 Summer Olympics and the 2016 Summer Paralympics, making the city the first South American and Portuguese-speaking city to ever host the events, and the third time the Olympics were 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.