The Immigration Act of 1891, also known as the 1891 Immigration Act, was a modification of the Immigration Act of 1882, focusing on immigration rules and enforcement mechanisms for foreigners arriving from countries other than China. It was the second major federal legislation related to the mechanisms and authority of immigration enforcement, the first being the Immigration Act of 1882 (there were other, more minor pieces of legislation passed in the 1880s). The law was passed on March 3, 1891, at the end of the term of the 51st United States Congress, and signed into law by then United States President Benjamin Harrison.

The Immigration Act of 1882 was the first major federal legislation describing a framework for regulation of immigration. The Act allowed for head taxes on certain arriving migrants, and allowed the use of this to fund a federal immigration bureaucracy.

A parallel set of legislative, executive, and judicial changes was unfolding in the domain of Chinese exclusion. The key pieces of legislation were:

The Act specified that the officers in charge of any vessel arriving by sea had to submit a list of passengers with their biographical information to the immigration inspectors at the port. Requirements of this sort had been part of United States federal law since the requirement for a manifest of immigrants in Section 4 of the Steerage Act of 1819. However, in this case the list needed to be submitted immediately upon arrival and was used to inspect aliens prior to admitting them. The inspectors at ports of entry had the authority to conduct a medical examination of aliens suspected of being unfit or having dangerous diseases, marking the beginning of medical exclusion of immigrants in the United States. Aliens who were detained for a medical examination were still considered to not have formally entered the United States.

Whereas the Immigration Act of 1882 had only regulated the coastal borders of the United States, the Immigration Act of 1891 extended this authority to land borders with Canada and Mexico. It directed the Secretary of the Treasury to prescribe rules for the enforcement along land borders that would balance the interests of immigration law enforcement with the goal to not obstruct or unnecessarily delay, impede, or annoy passengers in ordinary travel between the countries.

An Act of October 19, 1888 had established the authority to remove migrants who should not have been allowed to enter for up to one year after their arrival. The Immigration Act of 1891 reiterated the authority to remove aliens. The following two provisions were relevant:

In addition to Sections 10 and 11, that pushed the financial responsibility for deporting an alien onto the vessel or persons who had brought the alien in, other provisions aimed to penalize the abetting of migration. Sections 3 and 4 forbade advertising journeys to migrants as a way of getting jobs, updating the Alien Contract Labor Law. Section 5 exempted ministers, professors, and others in skilled professions, as well as relatives of United States citizens, from the provisions of Sections 3 and 4.

The first challenge to the Act was Nishimura Ekiu v. United States. Nishimura Ekiu, a female citizen of Japan, arrived at the United States from Yokohama, Japan on May 7, 1891, and was denied entry under the Immigration Act of 1891. She sued for habeas corpus, and challenged the constitutionality of some provisions of the Immigration Act of 1891. The case was decided against Ekiu and in favor of the United States.