The Sino-British Joint Declaration on the Question of Hong Kong was signed by the Prime Ministers of the People's Republic of China (PRC) and the United Kingdom (UK) governments on December 19, 1984 in Beijing.

The Declaration entered into force with the exchange of instruments of ratification on May 27, 1985, and was registered by the PRC and UK governments at the United Nations on June 12, 1985. In the Joint Declaration, the PRC Government stated that it had decided to resume the exercise of sovereignty over Hong Kong (including Hong Kong Island , Kowloon , and the New Territories ) with effect from July 1, 1997, and the UK Government declared that it would restore Hong Kong to the PRC with effect from July 1, 1997. The PRC Government also declared its basic policies regarding Hong Kong in the document.

In accordance with the "One Country, Two Systems " Principle agreed between the UK and the PRC, the socialism system of PRC shall not be practised in the Hong Kong Special Administrative Region (HKSAR), and Hong Kong's previous capitalist system and its way of life shall remain unchanged for a period of 50 years. The Joint Declaration provides that these basic policies shall be stipulated in the Hong Kong Basic Law.

The Joint Declaration is unlike an agreement made within a state, and the involvement of the UN creates obligations that internationalize the status of Hong Kong . The Joint Declaration is, therefore, not a private agreement between the PRC and the UK , but an international one.

International treaties are concluded between states, not within a state, and are usually not needed to be registered with the UN. However, the UN Secretary General could agree to be involved, if invited.

By registering the Joint Declaration with the UN, both the PRC and UK governments acknowledged a need for a neutral third party, and accepted its possible participation by doing so. Otherwise, they could have simply made it a bilateral treaty, which they did not choose to.

The background of the Sino-British Joint declaration was the pending expiration of the lease of the New Territories on July 1, 1997. The lease was negotiated between the UK and the Guangxu Emperor of China, and was for a period of 99 years starting from 1898. At the time of the lease signing, the Hong Kong Island had already been ceded to the UK in perpetuity under the Treaty of Nanking in 1842 after the First Opium War, and the Kowloon Peninsula had also been ceded to the UK in perpetuity under the Convention of Peking in 1860 after the Second Opium War.