Adelaide ( (listen) AD-il-ayd; Kaurna: Tarntanya) is the capital city of South Australia, the state's largest city and the fifth-most populous city in Australia. "Adelaide" may refer to either Greater Adelaide (including the Adelaide Hills) or the Adelaide city centre. The demonym Adelaidean is used to denote the city and the residents of Adelaide. The Traditional Owners of the Adelaide region are the Kaurna people. The area of the city centre and surrounding parklands is called Tarndanya in the Kaurna language.

Adelaide is situated on the Adelaide Plains north of the Fleurieu Peninsula, between the Gulf St Vincent in the west and the Mount Lofty Ranges in the east. Its metropolitan area extends 20 km (12 mi) from the coast to the foothills of the Mount Lofty Ranges, and stretches 96 km (60 mi) from Gawler in the north to Sellicks Beach in the south.

Named in honour of Queen Adelaide, 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 centre and chose its location close to the River Torrens. Light's design, now listed as national heritage, set out the city centre in a grid layout known as "Light's Vision", interspaced by wide boulevards and large public squares, and entirely surrounded by parklands.

Early colonial Adelaide was shaped by the diversity and wealth of its free settlers, in contrast to the convict history of other Australian cities. Until the post-war era, it was Australia's third most populated city. It has been noted for its leading examples of religious freedom and progressive political reforms, and became known as the "City of Churches" due to its diversity of faiths. Today, Adelaide is known by its many festivals and sporting events, its food and wine, its coastline and hills, and its large defence and manufacturing sectors. Adelaide's quality of life has ranked consistently highly in various measures through the 21st century, at one stage being named Australia's most liveable city.

As South Australia's 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 boulevards of North Terrace and King William Street.