Lend-Lease, formally the Lend-Lease Act and introduced as An Act to Promote the Defense of the United States (Pub. L. 77–11, H.R. 1776, 55 Stat. 31, enacted March 11, 1941), was a policy under which the United States supplied the United Kingdom, the Soviet Union, France, the Republic of China, and other Allied nations of the Second World War with food, oil, and materiel between 1941 and 1945. The aid was given free of charge on the basis that such help was essential for the defense of the United States.

The Lend-Lease Act was signed into law on March 11, 1941, and ended on September 20, 1945. A total of $50.1 billion (equivalent to $690 billion in 2024 when accounting for inflation) worth of supplies was shipped, or 17% of the total war expenditures of the U.S. In all, $31.4 billion went to the United Kingdom, $11.3 billion to the Soviet Union, $3.2 billion to France, $1.6 billion to China, and the remaining $2.6 billion to other Allies. Roosevelt's top foreign policy advisor Harry Hopkins had effective control over Lend-Lease, making sure it was in alignment with Roosevelt's foreign policy goals.

Materiel delivered under the act was supplied at no cost, to be used until returned or destroyed. In practice, most equipment was destroyed, although some hardware (such as ships) was returned after the war. Supplies that arrived after the termination date were sold to the United Kingdom at a large discount for £1.075 billion, using long-term loans from the United States, which were finally repaid in 2006. Similarly, the Soviet Union repaid $722 million in 1971, with the remainder of the debt written off.

Reverse Lend-Lease to the United States totalled $7.8 billion. Of this, $6.8 billion came from the British and the Commonwealth. Canada also aided the United Kingdom and other Allies with the Billion Dollar Gift and Mutual Aid totalling $3.4 billion in supplies and services (equivalent to $61 billion in 2020).

Lend-Lease ended the United States' neutrality which had been enshrined in the Neutrality Acts of the 1930s. It was a decisive step away from non-interventionist policy and toward open support for the Allies. Lend-Lease's precise significance to Allied victory in World War II is debated. Khrushchev claimed that Stalin told him that Lend-Lease enabled the Soviet Union to defeat Germany.

The 1930s began with one of the world's greatest economic depressions, and the later recession of 1937–1938 (although minor relative to the Great Depression) was otherwise also one of the worst of the 20th century. In 1934, following the Nye Committee hearings, as well as the publication of influential books such as Merchants of Death, the United States Congress adopted several Neutrality Acts in the 1930s, motivated by non-interventionism—following the aftermath of its costly involvement in World War I (the war debts were still not paid off), and seeking to ensure that the country would not become entangled in foreign conflicts again. The Neutrality Acts of 1935, 1936, and 1937 intended to keep the United States out of war by making it illegal for Americans to sell or transport arms or other war materials to warring nations, be they aggressors or defenders.

In 1939, however—as Germany, Japan, and Italy pursued aggressive, militaristic policies—President Roosevelt wanted more flexibility to help contain Axis aggression. He suggested amending the act to allow warring nations to purchase military goods, arms and munitions if they paid cash and bore the risks of transporting the goods on non-American ships, a policy that would favor Britain and France. Initially, this proposal failed, but after Germany and the Soviet Union invaded Poland in September, Congress passed the Neutrality Act of 1939 ending the munitions embargo on a "cash and carry" basis. The passage of the 1939 amendment to the previous Neutrality Acts marked the beginning of a congressional shift away from isolationism, making a first step toward interventionism.

After the Fall of France during June 1940, the British Commonwealth and Empire were the only forces engaged in war against Germany and Italy, until the Italian invasion of Greece. Britain had been paying for its materiel with gold as part of the "cash and carry" program, as required by the U.S. Neutrality Acts of the 1930s, but by 1941 it had liquidated a large part of its overseas holdings and its gold reserves were becoming depleted in paying for materiel from the United States.