The United Nations Conference on International Organization (UNCIO), commonly known as the San Francisco Conference, was a convention of delegates from 50 Allied nations that took place towards the end of World War II, from 25 April to 26 June 1945 in San Francisco, California, United States. At this convention, the delegates reviewed and rewrote the Dumbarton Oaks agreements of the previous year. The convention resulted in the creation of the United Nations Charter, which was opened for signature on 26 June, the last day of the conference. The conference was held at various locations, primarily the War Memorial Opera House, with the Charter being signed on 26 June at the Herbst Theatre in the Veterans Building, part of the Civic Center. A square adjacent to the Civic Center, called "UN Plaza", commemorates the conference.

Allied ideas for the postwar world appeared in the 1941 London Declaration, although the Allies, including the United States, had been planning for the post-war period for some time already. The idea for Four Policemen was the vision of US President Franklin Roosevelt in which the United States, the United Kingdom, the Soviet Union, and China would lead the post-World War II international order. These countries, with the addition of France, would assume the permanent seats on the United Nations Security Council. At the February 1945 conference in Malta, it was proposed that the permanent members have veto power. This proposal was adopted shortly after at the Yalta Conference. While at Yalta, they began sending invitations to the San Francisco conference on international organization. A total of 46 countries were invited to San Francisco, all of which had declared war on Germany and Japan, having signed the Declaration by United Nations.

The conference directly invited four additional countries: Denmark (newly liberated from Nazi occupation), Argentina and the Soviet republics of Belarus and Ukraine. The participation of these countries was not without controversy. The decision on the participation of Argentina was troubled because of Soviet opposition to Argentine membership, arguing that Argentina had supported the Axis Powers during the war. Several Latin American countries opposed the inclusion of Belarus and Ukraine unless Argentina was admitted. In the end, Argentina was admitted to the conference with support from the United States and the desire for the participation of the Soviet Union at the conference was maintained.

The participation of Belarus and Ukraine at the conference came as a result of Roosevelt and Churchill's concession to Joseph Stalin, the Soviet leader who had originally requested all republics of the Soviet Union to have membership in the United Nations, but the US government launched a counterproposal in which all US states would obtain membership in the United Nations. The counterproposal encouraged Stalin to attend the Yalta Conference by accepting only Ukraine and Belarus's admission to the United Nations. That was intended to ensure a balance of power within the United Nations, which, in the opinion of the Soviets, was unbalanced towards the Western countries. For that purpose, modifications were made to the constitutions of the two republics in question so that Belarus and Ukraine's international legal subjects were limited while they were still part of the Soviet Union.^{[citation needed]}

Poland, despite having signed the Declaration by United Nations, did not attend the conference because there was no consensus on the formation of the postwar Polish government. Therefore, a space was left blank for the Polish signature. The new Polish government was formed after the conference (28 June) and signed the United Nations Charter on 15 October, which made Poland one of the founding countries of the United Nations.

On 25 April 1945, the conference started in San Francisco. 850 delegates, along with advisors, employees and staff of the secretariat, attended the conference, totalling 3,500 attendees. In addition, the conference was attended by 2,500 representatives of the media and observers from numerous organizations and societies. Earl Warren, the Governor of California, set the tone for the conference in his welcome speech:

We recognize that our future is linked with a world future in which the term "good neighbor" has become a global consideration. We have learned that understanding of one another’s problems is the greatest assurance of peace. And that true understanding comes only as a product of free consultation. This conference is proof in itself of the new conception of neighborliness and unity which must be recognized in world affairs.

Due to the fact that President Roosevelt, who was supposed to host the conference, died on 12 April 1945, the delegates held a commemorative ceremony in Muir Woods National Monument Cathedral Grove to place a comparative plaque for him.