The Open Door Policy (Chinese: ) 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 them from "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.

John Milton Hay (October 8, 1838 – July 1, 1905) was an American statesman and official whose career in government stretched over almost half a century. Beginning as a private secretary and assistant to Abraham Lincoln, Hay's highest office was United States Secretary of State under Presidents William McKinley and Theodore Roosevelt. Hay was also an author and biographer, and wrote poetry and other literature throughout much of his life.

Born in Indiana to an anti-slavery family that moved to Warsaw, Illinois when he was young, Hay showed great potential, and his family sent him to Brown University. After graduation in 1858, Hay read law in his uncle's office in Springfield, Illinois, adjacent to that of Lincoln. Hay worked for Lincoln's successful presidential campaign and became one of his private secretaries at the White House. Throughout the American Civil War, Hay was close to Lincoln and stood by his deathbed after the President was shot at Ford's Theatre. In addition to his other literary works, Hay co-authored with John George Nicolay a multi-volume biography of Lincoln that helped shape the assassinated president's historical image.

After Lincoln's death, Hay spent several years at diplomatic posts in Europe, then worked for the New-York Tribune under Horace Greeley and Whitelaw Reid. Hay remained active in politics, and from 1879 to 1881 served as Assistant Secretary of State. Afterward, he remained in the private sector, until President McKinley, for whom he had been a major backer, made him Ambassador to the United Kingdom in 1897. Hay became Secretary of State the following year.

Hay served for almost seven years as Secretary of State, under President McKinley, and after McKinley's assassination, under Theodore Roosevelt. Hay was responsible for negotiating the Open Door Policy, which kept China open to trade with all countries on an equal basis, with international powers. By negotiating the Hay–Pauncefote Treaty with the United Kingdom, the (ultimately unratified) Hay–Herrán Treaty with Colombia, and finally the Hay–Bunau-Varilla Treaty with the newly independent Republic of Panama, Hay also cleared the way for the building of the Panama Canal.