Macau or Macao is a special administrative region of the People's Republic of China (PRC). With a population of about 710,000 people and a land area of 32.9 km² (12.7 sq mi), it is the most densely populated region in the world.

Formerly a Portuguese colony, the territory of Portuguese Macau was first leased to Portugal by the Ming dynasty as a trading post in 1557. Portugal paid an annual rent and administered the territory under Chinese sovereignty until 1887, when Portugal gained perpetual colonial rights with the signing of the Sino-Portuguese Treaty of Peking. The colony remained under Portuguese rule until the 1999 handover to China. Macau is a special administrative region of China, which maintains separate governing and economic systems from those of mainland China under the principle of "one country, two systems". The unique blend of Portuguese and Chinese architecture in the city's historic centre has resulted in its designation as a UNESCO World Heritage Site in 2005.

The gambling industry of Macau is seven times larger than that of Las Vegas. The city has one of the highest GDPs per capita and GDPs per capita by purchasing power parity in the world.

It has a very high Human Development Index of 0.934, as calculated by the Government of Macau, and has the third-highest life expectancy in the world. The territory is highly urbanised, holding the status of the most densely populated territory on Earth; two-thirds of the total land area is built on land reclaimed from the sea.

The first known written record of the name Macau, rendered as A Ma Gang (亞/阿-媽/馬-港), is found in a letter dated 20 November 1555. The local inhabitants believed that the sea goddess Matsu (alternatively called A-Ma) had blessed and protected the harbour and referred to the waters around A-Ma Temple by her name. When Portuguese explorers first arrived in the area and asked for the place name, the locals thought they were asking about the temple and told them it was Ma Kok (媽閣). The earliest Portuguese spelling for this was Amaquão. Multiple variations were used until Amacão / Amacao and Macão / Macao became common during the 17th century.

The 1911 reform of Portuguese orthography standardised the spelling as Macau; however, the use of Macao persisted in English and other European languages.

The Macau Peninsula had many names in Chinese, including Jing'ao (井澳/鏡澳), Haojing (濠鏡), and Haojing'ao (濠鏡澳). The islands Taipa, Coloane, and Hengqin were collectively called Shizimen (十字門). These names would later become Oumún (澳門 'bay gate' or 'port gate', Mandarin Àomén), referring to the whole territory.

During the Qin dynasty (221–206 BC), the region was under the jurisdiction of Panyu County, Nanhai Prefecture of the province of Guangdong. The region is first known to have been settled during the Han dynasty. It was administratively part of Dongguan Prefecture in the Jin dynasty (266–420 AD), and alternated under the control of Nanhai and Dongguan in later dynasties. In 1152, during the Song dynasty (960–1279 AD), it was under the jurisdiction of the new Xiangshan County. In 1277, approximately 50,000 refugees fleeing the Mongol conquest of China settled in the coastal area.