"One country, two systems" is a constitutional principle of the People's Republic of China describing the governance of Hong Kong and Macau since they became Special Administrative Regions (SARs) of China in 1997 and 1999 respectively.

The constitutional principle was formulated in the early 1980s during negotiations over Hong Kong between China and the United Kingdom. It provided that there would be only one China, but that these regions could retain their own economic and administrative systems, while the rest of Mainland China uses the socialism with Chinese characteristics system. Under the principle, each of the two regions could continue to have its own governmental system, legal, economic and financial affairs, including trade relations with foreign countries, all of which are independent from those of the Mainland. The PRC has also proposed to apply the principle in the unification it aims for with Taiwan.

Since the passing of the National Security Law by Hong Kong in July 2020 however, many observers such as the United States have claimed that Hong Kong has reverted to a de facto "one country, one system." Additionally, the Hong Kong Bar Association and a number of governments have disputed the constitutionality of the law.