Nairobi is the capital and largest city of Kenya. The city and its surrounding area also forms the Nairobi County. The name "Nairobi" comes from the Maasai phrase Enkare Nairobi, which translates to "cold water," the Maasai name of the Nairobi river, which in turn lent its name to the city. However, it is popularly known as the "Green City in the Sun" and is surrounded by several expanding villa suburbs. Residents of Nairobi are known as Nairobians.

Founded by the British in 1899 as a simple rail depot on the railway linking Mombasa to Uganda, the town quickly grew to become the capital of British East Africa in 1907 and eventually the capital of a free Kenyan republic in 1963. During Kenya's colonial period, the city became a centre for the colony's coffee, tea and sisal industry. Nairobi is also the capital of the Nairobi Province and of the Nairobi District. The city lies on the Nairobi River, in the south of the nation, and has an elevation of 1795 m above sea-level.

Nairobi is the most populous city in East Africa, with a current estimated population of about 3 million. According to the 2009 Census, in the administrative area of Nairobi, 3,138,295 inhabitants lived within 696 km2 (269 sq mi). Nairobi is currently the 12th largest city in Africa, including the population of its suburbs.

Nairobi is now one of the most prominent cities in Africa politically and financially. Home to thousands of Kenyan businesses and over 100 major international companies and organisations, including the United Nations Environment Programme (UNEP) and the main coordinating and headquarters for the UN in Africa & Middle East, the United Nations Office in Nairobi (UNON), Nairobi is an established hub for business and culture. The Nairobi Stock Exchange (NSE) is one of the largest in Africa and the second oldest exchange on the continent. It is ranked 4th in terms of trading volume and capable of making 10 million trades a day. The globalisation and World Cities Study Group and Network (GaWC) defines Nairobi as a prominent social centre.