The Declaration by the United Nations was a World War II document agreed on 1 January 1942 during the Arcadia Conference by 26 governments: the Allied "Big Four'' (the US, the UK, the USSR, and China), nine other American countries in North and Central America and the Caribbean, the four British Dominions, British India, and eight Allied governments-in-exile, for a total of twenty-six nations.

The Declaration by United Nations, on 1 January 1942, was the basis of the modern UN.

The earliest concrete plan for a new world organization began under the aegis of the US State Department in 1939. The text of the "Declaration by United Nations" was drafted by President Franklin Roosevelt, British Prime Minister Winston Churchill, and Roosevelt aide Harry Hopkins, while meeting at the White House on 29 December 1941. It incorporated Soviet suggestions, but left no role for France. Roosevelt first coined the term United Nations to describe the Allied countries. Roosevelt suggested "United Nations" as an alternative to the name "Associated Powers." Churchill accepted it, noting that the phase was used by Lord Byron in the poem Childe Harold's Pilgrimage (Stanza 35). The term was first officially used on 1–2 January 1942, when 26 governments signed the Declaration. One major change from the Atlantic Charter was the addition of a provision for religious freedom, which Stalin approved after Roosevelt insisted. By spring 1945 it was signed by 21 more states.

The Declaration by United Nations, on 1 January 1942, was the basis of the modern UN. The term United Nations became synonymous during the war with the Allies and was considered to be the formal name that they were fighting under. The text of the declaration affirmed the signatories' perspective "that complete victory over their enemies is essential to defend life, liberty, independence and religious freedom, and to preserve human rights and justice in their own lands as well as in other lands, and that they are now engaged in a common struggle against savage and brutal forces seeking to subjugate the world". The principle of "complete victory" established an early precedent for the Allied policy of obtaining the Axis' powers' "unconditional surrender". The defeat of "Hitlerism" constituted the overarching objective, and represented a common Allied perspective that the totalitarian militarist regimes ruling Germany, Italy, and Japan were indistinguishable.The declaration, furthermore, "upheld the Wilsonian principles of self determination," thus linking U.S. war aims in both world wars.

By the end of the war, 21 other states had acceded to the declaration, including the Philippines, France, every Latin American state except Argentina, and the various independent states of the Middle East and Africa. Although most of the minor Axis powers had switched sides and joined the United Nations as co-belligerents against Germany by the end of the war, they were not allowed to accede to the declaration. Occupied Denmark did not sign the declaration, but because of the vigorous resistance after 1943, and because the Danish ambassador Henrik Kauffmann had expressed the adherence to the declaration of all free Danes, Denmark was nonetheless invited among the allies in the San Francisco Conference in March 1945.