Adelaide (pronounced /ˈædəleɪd/) is the capital and most populous city of the Australian state of South Australia, and is the fifth-largest city in Australia, with a population of more than 1.1 million. It is a coastal city situated on the eastern shores of Gulf St. Vincent, on the Adelaide Plains, north of the Fleurieu Peninsula, between the Gulf St. Vincent and the low-lying Mount Lofty Ranges. It is roughly 20 km (12 mi) from the coast to the foothills but sprawls 90 km (56 mi) from Gawler at its northern extent to Sellicks Beach in the south.

Named in honour of Queen Adelaide who was born in Germany, the consort of King William IV, the city was founded in 1836 as the planned capital for the only freely-settled British province in Australia. Colonel William Light, one of Adelaide's founding fathers, designed the city and chose its location close to the River Torrens in the area originally inhabited by Indigenous Australians of the Kaurna tribe. Light's design set out Adelaide in a grid layout, interspaced by wide boulevards and large public squares, and entirely surrounded by parkland. Early Adelaide was shaped by religious freedom and a commitment to political progressivism and civil liberties, which led to world-first reforms.

As South Australia's seat of government and commercial centre, Adelaide is the site of many governmental and financial institutions. Most of these are concentrated in the city centre along the cultural boulevard of North Terrace, King William Street and in various districts of the metropolitan area.

Today, Adelaide is noted for its many festivals and sporting events, its food, wine and culture, its long beachfronts, and its large defence and manufacturing sectors. It continues to rank highly as a livable city, being in the Top 10 in The Economist's World's Most Livable Cities index.