Baku (US: , UK: ; Azerbaijani: Bakı [bɑˈcɯ] (listen)) is the capital and largest city of Azerbaijan, as well as the largest city on the Caspian Sea and of the Caucasus region. Baku is 28 metres (92 ft) below sea level, which makes it the lowest lying national capital in the world and also the largest city in the world below sea level. Baku lies on the southern shore of the Absheron Peninsula, on the Bay of Baku. Baku's urban population was estimated at two million people as of 2009. Baku is the primate city of Azerbaijan—it is the sole metropolis in the country, and about 25% of all inhabitants of the country live in Baku's metropolitan area.

Baku is divided into twelve administrative raions and 48 townships. Among these are the townships on the islands of the Baku Archipelago, as well as the industrial settlement of Neft Daşları built on oil rigs 60 kilometres (37 miles) away from Baku city in the Caspian Sea. The old city, containing the Palace of the Shirvanshahs and the Maiden Tower, was designated as a UNESCO World Heritage Site in 2000.

The city is the scientific, cultural, and industrial centre of Azerbaijan. Many sizeable Azerbaijani institutions have their headquarters there. In the 2010s, Baku became a venue for major international events. It hosted the 57th Eurovision Song Contest in 2012, the 2015 European Games, 4th Islamic Solidarity Games, the European Grand Prix in 2016, the Azerbaijan Grand Prix since 2017, the final of the 2018–19 UEFA Europa League, and was one of the host cities for UEFA Euro 2020. The Baku International Sea Trade Port is capable of handling two million tonnes of general and dry bulk cargoes per year. Baku is renowned for its harsh winds, reflected in its nickname, the "City of Winds".