The Nationality Act of 1940 (H.R. 9980; Pub.L. 76-853; 54 Stat. 1137) revised numerous provisions of law relating to American citizenship and naturalization. It was enacted by the 76th Congress of the United States and signed into law on October 14, 1940, a year after World War II had begun in Europe, but before the U.S. entered the war.

The law revised "the existing nationality laws of the U.S. into a more complete nationality code"; it defined those persons who were "eligible for citizenship through birth or naturalization" and clarified "the status of individuals and their children born or residing in the continental U.S., its territories such as Alaska, Hawaii, Puerto Rico, the Virgin Islands, the Philippines, Panama and the Canal Zone, or abroad." The law furthermore defined who was not eligible for citizenship, and how citizenship could be lost or terminated. This legislation represents the first attempt ever made, since the founding of the United States, to codify and unify all of the U.S. laws relating to nationality and naturalization.

This act was superseded by the Immigration and Nationality Act of 1952 and by later acts of Congress and executive orders.

The law is divided into several chapters, as follows:

The Act has five chapters to it, each having provisions to what the process was to gain citizenship and to not lose it.

Chapter one of the act outlines the definition of terms used in the Act.

Chapters two and three are the largest parts of the Act and they deal with identifying eligibility for citizenship and specific residency requirements for people born abroad to one U.S. citizen parent, or non-citizens born in the U.S. or its territories.

The third chapter of the act goes into detail about nationality through naturalization, giving a further clarification of requirements for non-citizens seeking naturalization. It goes into detail with specifications concerning race, ethnicity, and basic verbal English proficiency along with residency requirements outlined.