The Immigration Act of 1924, or Johnson–Reed Act, including the National Origins Act, and Asian Exclusion Act (Pub.L. 68–139, 43 Stat. 153, enacted May 26, 1924), was a United States federal law that set quotas on the number of immigrants from certain countries while providing funding and an enforcement mechanism to carry out the longstanding (but hitherto unenforced) ban on other non-white immigrants. The law was primarily aimed at further decreasing immigration of Southern Europeans, especially Italians; and, to a lesser extent, immigrants from countries with Roman Catholic majorities, Eastern Europeans, Arabs, and Jews. The law affirmed the longstanding ban on the immigration of other non-white persons, with the exception of black African immigrants (who had long been exempt from the ban). Thus, virtually all Asians were forbidden from immigrating to America under the Act. Subsequent court rulings would determine that Indians and Arabs were not white and could not immigrate.

Contrary to popular belief, Latin Americans were not prohibited or limited from immigrating under the law. In most states and under federal law, persons of mixed white and Native American ancestry were considered white; this principle was interpreted under the Act to allow Latin Americans to immigrate as “white persons.” Moreover, unlike Eastern and Southern Europe, no nationality-based quotas were placed on Latin American immigrants. Thus, the law allowed unlimited Latin American immigration, just as it allowed unlimited northwestern European immigration. Ironically, the 1965 immigration law that replaced the 1924 Act, though abolishing racial preferences and national quotas, would effectively place greater restriction on Latin American immigration.

According to the U.S. Department of State Office of the Historian the purpose of the act was “to preserve the ideal of American homogeneity”. But though the Act aimed at preserving American racial homogeneity, it set no limits on immigration from other countries of the Americas, including Latin America. Congressional opposition was minimal. According to Columbia University historian Mae Ngai, the 1924 Act put an end to a period where the United States essentially had open borders.

A key element of the Act was its provisions for enforcement. The longstanding ban on nonwhite immigration to the US and the popular ire against Italian immigration had rarely been officially enforced in practice at that point. The Act instead provided funding and legal instructions to courts of deportation for non-white immigrants and Southern and Eastern European immigrants who exceeded their national quotas. Deportations of North African, Arabian, Persian, and East Indian immigrants were sometimes challenged in court with claims that these persons were “white.” Initially these immigrants were deported, but by the post-World War II era, with xenophobia on the decline, all of these groups were classified as white under American law.