The Administrative Procedure Act (APA), Pub. L. 79–404, 60 Stat. 237, enacted June 11, 1946, is the United States federal statute that governs the way in which administrative agencies of the federal government of the United States may propose and establish regulations, and it grants U.S. federal courts oversight over all agency actions. According to Hickman & Pierce, it is one of the most important pieces of United States administrative law, and serves as a sort of "constitution" for U.S. administrative law.

The APA applies to both the federal executive departments and the independent agencies. U.S. senator Pat McCarran called the APA "a bill of rights for the hundreds of thousands of Americans whose affairs are controlled or regulated" by federal government agencies. The text of the APA can be found under Title 5 of the United States Code, beginning at Section 500. Section 702 of the APA waives sovereign immunity, allowing people to sue a federal agency in court for non-monetary relief, such as an injunction or a declaratory judgment.

There is a similar Model State Administrative Procedure Act (Model State APA), which was drafted by the National Conference of Commissioners on Uniform State Laws for oversight of state agencies. Not all states have adopted the model law wholesale, as of 2017. The federal APA does not require systematic oversight of regulations prior to adoption, unlike the Model APA. Each US state has passed its own version of the Administrative Procedure Act.

Beginning in 1933, President Franklin D. Roosevelt and the Democratic Congress enacted several statutes that created new federal agencies as part of the New Deal legislative plan, established to guide the United States through the social and economic hardship caused by the Great Depression. However, the Congress became concerned about the expanding powers that these autonomous federal agencies now possessed, resulting in the enactment of the APA to regulate, standardize and oversee these federal agencies.

The APA was born in a contentious political environment. Professor George Shepard claims that Roosevelt's opponents and supporters fought over passage of the APA "in a pitched political battle for the life of the New Deal" itself. Shepard notes, however, that a legislative balance was struck with the APA, expressing "the nation's decision to permit extensive government, but to avoid dictatorship and central planning."

A 1946 House of Representatives report discusses the 10-year period of "painstaking and detailed study and drafting" that went into the APA. Because of rapid growth in the administrative regulation of private conduct, Roosevelt ordered several studies of administrative methods and conduct during the early part of his four-term presidency. Based on one study, Roosevelt commented that the practice of creating administrative agencies with the authority to perform both legislative and judicial work "threatens to develop a fourth branch of government for which there is no sanction in the Constitution".

In 1939, Roosevelt requested for Attorney General Frank Murphy to form a committee to investigate practices and procedures in American administrative law and suggest improvements. That committee's report, the Final Report of Attorney General's Committee on Administrative Procedure, contained detailed information about the development and procedures of the federal agencies.

The Final Report defined a federal agency as a governmental unit with "the power to determine ... private rights and obligations" by rulemaking or adjudication. The report applied that definition to the largest units of the federal government, and identified "nine executive departments and eighteen independent agencies". Overall, 51 federal agencies were identified in the report after including various subdivisions within the larger units. In reviewing the history of federal agencies, the Final Report noted that almost all agencies had undergone changes in name and political function.