The Open Door Policy is the maintenance in a certain territory of equal commercial and industrial rights for the nationals of all countries. As a specific policy, it was first advanced by the United States, but it was rooted in the typical most favored nation clause of the treaties concluded with China after the Opium War (1839-42). The Open Door Policy is the current stated policy of the government of the People's Republic of China starting in 1978 with the Chinese economic reform policies under Deng Xiaoping.

Although the Open Door is generally associated with China, it also received recognition at the Berlin Conference of 1885, which declared that no power could levy preferential duties in the Congo basin.

In 1898, the United States became an East Asian power through the acquisition of the Philippine Islands, and when the partition of China by the European powers and Japan seemed imminent, the U.S. government strove to preserve equal industrial and commercial privileges. Secretary of State John Hay sent (1899) notes to the major powers (France, Germany, the United Kingdom, Italy, Japan, and Russia), asking them to declare formally that they would uphold Chinese territorial and administrative integrity and would not interfere with the free use of the treaty ports within their spheres of influence in China. In replying, each nation evaded Hay's request, taking the position that it could not commit itself until the other nations had complied. However, in March 1900, Hay announced that the powers had granted consent to his request. Only Japan challenged this declaration, and the Open Door became an international policy.

After the Boxer Uprising, Hay dispatched (1900) a similar circular note.

Two years later, the U.S. government protested that Russian encroachment in Manchuria was a violation of the Open Door. When Japanese replaced Russian influence in Southern Manchuria after the Russo-Japanese War (1904-5) the Japanese and U.S. governments pledged to maintain a policy of equality in Manchuria. In finance, American efforts to preserve the Open Door led (1909) to the formation of an international banking consortium through which all Chinese railroad loans would be made. The United States withdrew in 1913, asserting that the consortium violated Chinese administrative integrity.

The next violation of the Open Door policy occurred in 1915, when Japan presented to China the Twenty-one Demands. That incident led (1917) to another exchange of notes between the United States and Japan in which there were renewed assurances that the Open Door would be respected, but that the United States recognized Japan's special interests in China. The Open Door principle had been further weakened by a series of secret treaties (1917) between Japan and the Allies, which promised Japan the German possessions in China.

The increasing disregard of the Open Door was a main reason for the convocation of the Conference on the Limitation of Armament (1921-22) in Washington, D.C. As a result of the conference, the Nine-Power Treaty , guaranteeing the integrity and independence of China and reaffirming the Open Door principle, was signed by the United States, the United Kingdom, Japan, France, Italy, the Netherlands, Portugal, China, and Belgium. With the Japanese seizure (1931) of Manchuria and the creation of Manchukuo, however, the Open Door received its greatest reverse.

After World War II, China's position as a sovereign state was recognized. No nation, therefore, had the right or capacity to carve out spheres of influence or to attempt to exclude other states from trade. With the rise to power of the Communist Party of China, the Open Door Policy was rejected until the late-1970's in which Deng Xiaoping committed China to adopting policies which promote foreign trade and economic investment. Since the late-1970's, the government of the People's Republic of China has maintained a policy of encouraging foreign trade.