Rio de Janeiro (January River), commonly referred to simply as Rio, is the capital city of the State of Rio de Janeiro, the second largest city of Brazil, and the third largest metropolitan area and agglomeration in South America, boasting approximately 6.3 million people within the city proper, making it the 6th largest in the Americas, and 26th in the world. Part of the city has been designated as a World Heritage Site, named "Rio de Janeiro: Carioca Landscapes between the Mountain and the Sea", identified by UNESCO on 1 July 2012 in the category Cultural Landscape.

The city was the capital of Brazil for nearly two centuries, from 1763 to 1815 during the Portuguese colonial era, 1815 to 1821 as the capital of the United Kingdom of Portugal, Brazil and Algarves, and 1822 to 1960 of Brazil as an independent nation. Rio is nicknamed the Cidade Maravilhosa or "Marvelous City".

Rio de Janeiro represents the second largest GDP in the country (and 30th largest in the world in 2008), estimated at about R$343 billion (IBGE/2008) (nearly US$201 billion), and is headquarters to two of Brazil's major companies—Petrobras and Vale, and major oil companies and telephony in Brazil, besides the largest conglomerate of media and communications companies in Latin America, the Globo Organizations. 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 production—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 celebrations, samba, Bossa Nova, balneario beaches such as Barra da Tijuca, Copacabana, Ipanema, and Leblon. Some of the most famous landmarks in addition to the beaches include the giant statue of Christ the Redeemer ("Cristo Redentor") atop Corcovado mountain, named one of the New Seven Wonders of the World; Sugarloaf mountain (Pão de Açúcar) with its cable car; the Sambódromo, a permanent grandstand-lined parade avenue which is used during Carnival; and Maracanã Stadium, one of the world's largest football stadiums.

The 2016 Summer Olympics and the Paralympics will take place in Rio de Janeiro, which will mark the first time a South American or a Portuguese-speaking nation hosts the event. It will be the third time the Olympics will be held in a Southern Hemisphere city. On 12 August 2012, at the 2012 Summer Olympics closing ceremony, Mayor Eduardo Paes received the Olympic Flag, via Jacques Rogge, from London Mayor Boris Johnson. Rio's Maracanã Stadium, which held the final of the 1950 FIFA World Cup, will host the final match of the 2014 FIFA World Cup. Rio de Janeiro also hosted the World Youth Day in 2013.

Rio de Janeiro is on a strip of Brazil's Atlantic coast, close to the Tropic of Capricorn, where the shoreline is oriented east–west. Facing largely south, the city was founded on an inlet of this stretch of the coast, Guanabara Bay (Baía de Guanabara), and its entrance is marked by a point of land called Sugar Loaf (Pão de Açúcar)—a "calling card" of the city.

The Centre (Centro), the core of Rio, lies on the plains of the western shore of Guanabara Bay. The greater portion of the city, commonly referred to as the North Zone (Zona Norte), extends to the northwest on plains composed of marine and continental sediments and on hills and several rocky mountains. The South Zone (Zona Sul) of the city, reaching the beaches fringing the open sea, is cut off from the Centre and from the North Zone by coastal mountains. These mountains and hills are offshoots of the Serra do Mar to the northwest, the ancient gneiss-granite mountain chain that forms the southern slopes of the Brazilian Highlands. The large West Zone (Zona Oeste), long cut off by the mountainous terrain, had been made accessible by new roads and tunnels by the end of the 20th century.

The population of the city of Rio de Janeiro, occupying an area of 1,182.3 square kilometres (456.5 sq mi), is about 6,000,000. The population of the greater metropolitan area is estimated at 11–13.5 million. It was Brazil's capital until 1960, when Brasília took its place. Residents of the city are known as cariocas. The official song of Rio is "Cidade Maravilhosa", by composer André Filho.

Rio de Janeiro has the second largest GDP of any city in Brazil, surpassed only by São Paulo. According to the IBGE, it was approximately US$ 316 billion in 2012, equivalent to 5.1% of the national total. The services sector comprises the largest portion of GDP (65.52%), followed by commerce (23.38%), industrial activities (11.06%) and agriculture (0.04%).

Benefiting from the federal capital position occupied by a long period (1763–1960), the town became a dynamic center administrative, financial, commercial and cultural. Greater Rio de Janeiro, as perceived by the IBGE, has a GDP of US$ 187.374.116.000, constituting the second largest hub of national wealth. Per capita GDP is US$ 11,786. It concentrates 68% of the state's economic strength and 7.91% of all goods and services produced in the country.

Taking into consideration the network of influence exerted by the urban metropolis (which covers 11.3% of the population), this share in GDP rises to 14.4%, according to a study released in October 2008 by the IBGE. For many years brings together the second largest industrial hub of Brazil, with oil refineries, shipbuilding industries, steel, metallurgy, petrochemical, gas, chemical, textile, printing, publishing, pharmaceutical, beverages, cement and furniture. However, the last decades indicated a sharp transformation in its economic profile, which is acquiring more and more shades of a major national hub of services and businesses. The Stock Exchange of Rio de Janeiro (BVRJ), which currently trades only government securities, was the first stock exchange founded in Brazil in 1845 and located in the central region.

Rio de Janeiro became an attractive place for companies to locate when it was the capital of Brazil, as important sectors of society and of the government were present in the city. The city was chosen as headquarters for state-owned companies such as Petrobras, Eletrobrás, Caixa Econômica Federal and Vale (which was privatized in the 1990s). After the transfer of the capital to Brasília, in 1960, it kept attracting more companies, especially after the discovery of oil in the Campos Basin, which produces most of the total oil production of Brazil. This made many oil and gas companies to be based in Rio de Janeiro, such as the Brazilian branches of Shell, EBX and Esso. The headquarters of BNDES, an important state institution, is also in Rio de Janeiro. The city is also the headquarters of large telecom companies, such as Intelig, Oi and Embratel.

Rio ranks second nationally in industrial production and second financial and service center, trailing only São Paulo. The city's industries produce processed foods, chemicals, petroleum products, pharmaceuticals, metal products, ships, textiles, clothing, and furniture. The service sector dominates the economy, however, and includes banking and the second most active stock market in Brazil, the Bolsa da Valores do Brasil. Tourism and entertainment are other key aspects of the city's economic life and the city is the nation's top tourist attraction for both Brazilians and foreigners.

Because it was once the national capital, Rio de Janeiro was chosen as the site for the headquarters of many private, national, multinational, and state corporations, even when their factories were located in other cities or states. Despite the transfer of the capital to Brasília, many of these headquarters remained within the Rio metropolitan area, including those of Petrobrás, the state oil company, and the National Economic and Social Development Bank, a federal investment bank.

A newer electronics and computer sector has been added to the older industries of metallurgy, engineering, and printing and publishing. Other manufacturing sectors focus on the production of shipyard-related materials, apparel and footwear, textiles, nonmetallic mineral products, food and beverages, chemicals, and pharmaceuticals. Construction, also an important activity, provides a significant source of employment for large numbers of unskilled workers and is buoyed by the number of seasonal residents who build second homes in the Greater Rio de Janeiro area.

To attract industry, the state government has designated certain areas on the outskirts of the city as industrial districts where infrastructure is provided and land sales are made under special conditions. Oil and natural gas from fields off the northern coast of Rio de Janeiro state are a major asset used for developing manufacturing activities in Rio's metropolitan area, enabling it to compete with other major cities for new investment in industry.

As with manufacturing, Rio is an important financial centre, second only to São Paulo in volume of business in financial markets and in banking. Its securities market, although declining in significance relative to São Paulo, is still of major importance. Owing to the proximity of Rio's port facilities, many of Brazil's export-import companies are headquartered in the city. In Greater Rio, which has one of the highest per capita incomes in Brazil, retail trade is substantial. Many of the most important retail stores are located in the Centre, but others are scattered throughout the commercial areas of the other districts, where shopping centres, supermarkets, and other retail businesses handle a large volume of consumer trade.

Major Brazilian entertainment and media organizations are based in Rio de Janeiro like Organizações Globo and also some of Brazil's major newspapers: Jornal do Brasil, O Dia, and Business Rio. Major international pharmaceutical companies have their Brazilian headquarters in Rio such as: Merck, Roche, Arrow, Darrow, Baxter, Mayne, and Mappel.

Rio de Janeiro is the 2nd richest city in Brazil, behind São Paulo and the 30th richest city in the world with a GDP of R$ 201,9 billion in 2010. The per capita income for the city was R$22,903 in 2007 (around US$14,630). According to Mercer's city rankings of cost of living for expatriate employees, Rio de Janeiro ranks 12th among the most expensive cities in the world in 2011, up from the 29th position in 2010, just behind São Paulo (ranked 10th), and ahead of London, Paris, Milan, and New York City. Rio also has the most expensive hotel rates in Brazil, and the daily rate of its five star hotels are the second most expensive in the world after only NYC.