Lagos (Nigerian English: ; Yoruba: Èkó) is the most populous city in Nigeria as well as Africa with an estimated population of 15.9 million in 2015. The estimated population for Lagos city was more than 24 million in 2022; and around 30 million for the Lagos metropolitan area, including the suburban area reaching far into the neighbouring Ogun State, thus making Lagos the most populous urban area in Africa. Lagos was the national capital of Nigeria until December 1991 following the government's decision to move their capital to Abuja in the centre of the country. Lagos is a major African financial centre and is the economic hub of Lagos State and Nigeria at large. The city has been described as the cultural, financial, and entertainment capital of Africa, and is a significant influence on commerce, entertainment, technology, education, politics, tourism, art, and fashion. Lagos is also among the top ten of the world's fastest-growing cities and urban areas. The megacity has the fourth-highest GDP in Africa and houses one of the largest and busiest seaports on the continent. The Lagos metropolitan area is a major educational and cultural centre in Sub Saharan Africa. Due to the large urban population and port traffic volumes, Lagos is classified as a Medium-Port Megacity.

Lagos emerged as a home to the Awori subgroup of the Yoruba of West Africa islands, which are contained in the present day Local Government Areas (LGAs) of Lagos Island, Eti-Osa, Amuwo-Odofin and Apapa. The islands are separated by creeks, fringing the southwest mouth of Lagos Lagoon, while being protected from the Atlantic Ocean by barrier islands and long sand spits such as Bar Beach, which stretch up to 100 km (62 mi) east and west of the mouth. Due to rapid urbanisation, the city expanded to the west of the lagoon to include areas in the present day Lagos Mainland, Ajeromi-Ifelodun, and Surulere. This led to the classification of Lagos into two main areas: the Island, which was the original city of Lagos, and the Mainland, which it has since expanded into. This city area was governed directly by the Federal Government through the Lagos City Council, until the creation of Lagos State in 1967, which led to the splitting of Lagos city into the present-day seven Local Government Areas (LGAs), and an addition of other towns (which now make up 13 LGAs) from the then Western Region to form the state.

However, the state capital was later moved to Ikeja in 1976, and the federal capital moved to Abuja in 1991. Even though Lagos is still widely referred to as a city, the present-day Lagos, also known as "Metropolitan Lagos", and officially as "Lagos Metropolitan Area" is an urban agglomeration or conurbation, consisting of 16 LGAs including Ikeja, the state capital of Lagos State. This conurbation makes up 37% of Lagos State total land area, but houses about 85% of the state's total population.

The population of Metropolitan Lagos is disputed. In the 2006 federal census data, the conurbation had a population of about 8 million people. However, the figure was disputed by the Lagos State Government, which later released its own population data, putting the population of Lagos Metropolitan Area at approximately 16 million. Daily, the Lagos area is growing by some 3,000 people or around 1.1 million annually, so the true population figure of the greater Lagos area in 2022 is roughly 28 million (up from some 23.5 million in 2018). Lagos may therefore have overtaken Kinshasa as Africa's most populous city. As of 2015, unofficial figures put the population of "Greater Metropolitan Lagos", which includes Lagos and its surrounding metro area, extending as far as into Ogun State, at approximately 21 million.