Macau, also spelled Macao and officially the Macao Special Administrative Region of the People's Republic of China, is a city and special administrative region of the People's Republic of China in the western Pearl River Delta by the South China Sea. With a population of about 680,000 and an area of 32.9 km², it is the most densely populated region in the world.
Macau is a former colony of the Portuguese Empire, after Ming China leased the territory as a trading post in 1557. Portugal paid an annual rent and administered the territory under Chinese sovereignty until 1887, when it gained perpetual colonial rights in the Sino-Portuguese Treaty of Peking. The colony remained under Portuguese rule until 1999, when it was transferred 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 let to its inscription on the UNESCO World Heritage List in 2005.