The Twenty-One Demands (Japanese: 対華21ヶ条要求, romanized: Taika Nijūikkajō Yōkyū; simplified Chinese: 二十一条; traditional Chinese: 二十一條; pinyin: Èrshíyī tiáo) was a set of demands made during the First World War by the Empire of Japan under Prime Minister Ōkuma Shigenobu to the government of the Republic of China on 18 January 1915. The secret demands would greatly extend Japanese control of China. Japan would keep the former German areas it had conquered at the start of World War I in 1914. It would be strong in Manchuria and South Mongolia. It would have an expanded role in railways. The most extreme demands (in section 5) would give Japan a decisive voice in finance, policing, and government affairs. The last part would make China in effect a protectorate of Japan, and thereby reduce Western influence. Japan was in a strong position, as the Western powers were in a stalemated world war with Germany. Britain and Japan had a military alliance since 1902, and in 1914 London had asked Tokyo to enter the war. Beijing published the secret demands and appealed to Washington and London. They were sympathetic and forced Tokyo to drop section 5. In the final 1916 settlement, Japan gave up its fifth set of demands. It gained a little in China, but lost a great deal of prestige and trust in Britain and the U.S.

The Chinese public responded with a spontaneous nationwide boycott of Japanese goods; Japan's exports to China fell drastically. Britain was affronted and no longer trusted Japan as an ally. With the First World War underway, Japan's position was strong and Britain's was weak; nevertheless, Britain (and the United States) forced Japan to drop the fifth set of demands that would have given Japan a large measure of control over the entire Chinese economy and ended the Open Door Policy. Japan and China reached a series of agreements which ratified the first four sets of goals on 25 May 1915.