The Open Door Policy (Chinese: 門戶開放政策) is the United States diplomatic policy established in the late 19th and early 20th century that called for a system of equal trade and investment and to guarantee the territorial integrity of Qing China. The policy was enunciated in US Secretary of State John Hay's Open Door Note, dated September 6, 1899 and circulated to the major European powers. In order to prevent the "carving of China like a melon," as they were doing in Africa, the Note asked the powers to keep China open to trade with all countries on an equal basis and called upon all powers, within their spheres of influence to refrain from interfering with any treaty port or any vested interest, to permit Chinese authorities to collect tariffs on an equal basis, and to show no favors to their own nationals in the matter of harbor dues or railroad charges. The policy was accepted only grudgingly, if at all, by the major powers, and it had no legal standing or enforcement mechanism. In July 1900, as the powers contemplated intervention to put down the violently anti-foreign Boxer uprising, Hay circulated a Second Open Door Note affirming the principles. Over the next decades, American policy-makers and national figures continued to refer to the Open Door Policy as a basic doctrine, and Chinese diplomats appealed to it as they sought American support, but critics pointed out that the policy had little practical effect.

The term "Open Door" also describes the economic policy initiated by Deng Xiaoping in 1978 to open China to foreign businesses that wanted to invest in the country. The policy set into motion the economic transformation of China. In the 20th and 21st centuries, scholars such as Christopher Layne in the neorealist school have generalized the use of the term to applications in 'political' open door policies and 'economic' open door policies of nations in general, which interact on a global or international basis.