The Washington Naval Treaty, also known as the Five-Power Treaty, was signed during 1922 among the major Allies of World War I, which agreed to prevent an arms race by limiting naval construction. It was negotiated at the Washington Naval Conference in Washington, D.C., from November 1921 to February 1922 and signed by the governments of the British Empire (including the United Kingdom, Canada, Australia, New Zealand, South Africa and India), United States, France, Italy, and Japan. It limited the construction of battleships, battlecruisers and aircraft carriers by the signatories. The numbers of other categories of warships, including cruisers, destroyers, and submarines, were not limited by the treaty, but those ships were limited to 10,000 tons displacement each.

The treaty was finalized on February 6, 1922. Ratifications of it were exchanged in Washington on August 17, 1923, and it was registered in the League of Nations Treaty Series on April 16, 1924.

Later conferences on naval arms limitation sought additional limitations of warship building. The terms of the Washington Naval Treaty were modified by the London Naval Treaty of 1930 and the Second London Naval Treaty of 1936. By the mid-1930s, Japan and Italy had renounced the treaties, while Germany renounced the Treaty of Versailles which had limited its navy. Naval arms limitation became increasingly difficult for the other signatories.

Immediately after World War I, Britain still had the world's largest and most powerful navy, followed by the United States and, more distantly, by Japan, France and Italy.^{[citation needed]} The British Royal Navy interned the defeated German High Seas Fleet in November 1918. The Allies had differing opinions concerning the final disposition of the Imperial German Navy, with the French and Italians wanting the German fleet divided between the victorious powers and the Americans and British wanting the ships destroyed. The negotiations became mostly moot after the German crews scuttled most of their ships on June 21, 1919.

News of the scuttling angered the French and the Italians, with the French particularly unimpressed with British explanations that the fleet guarding the Germans had then been away on exercises. Nevertheless, the British joined their allies in condemning the German actions, and no credible evidence emerged to suggest that the British had collaborated with the Germans with respect to the scuttling. The Treaty of Versailles, signed a week later on June 28, 1919, imposed strict limits on the sizes and numbers of warships which the newly-installed German government had the right to build and maintain.

The Americans, British, French, Italians, and Japanese had been allies during World War I, but with the German threat seemingly finished, a naval arms race between the allies seemed likely. US President Woodrow Wilson's administration had already announced successive plans for the expansion of the US Navy from 1916 to 1919 that would have resulted in a massive fleet of 50 modern battleships.

In response, the Japanese Diet in 1920 finally authorised construction of warships to enable the Imperial Japanese Navy to attain its goal of an "eight-eight" fleet programme, with eight modern battleships and eight battlecruisers. The Japanese started work on four battleships and four battlecruisers, all of which were much larger and more powerful than those of the classes that they were replacing.

The 1921 British Naval Estimates planned four battleships and four battlecruisers, with another four battleships to follow the subsequent year.