The United States Border Patrol (USBP) is a federal law enforcement agency under the United States Customs and Border Protection (CBP) and is responsible for securing the borders of the United States. According to its website as of 2022, its mission is to "Protect the American people, safeguard our borders, and enhance the nation’s economic prosperity."

With 19,648 agents in 2019, the Border Patrol is one of the largest law enforcement agencies in the United States. For fiscal year 2017, Congress enacted a budget of $3,805,253,000 for the Border Patrol.

In the nineteenth century, United States borders were open and unrestricted; there was no systematic control or even recordkeeping of immigrants. The first legislation restricting immigration, after the Act Prohibiting Importation of Slaves of 1807, was the Chinese Exclusion Act of 1882.

Mounted watchmen of the United States Department of Commerce and Labor patrolled the border in an effort to prevent illegal crossings as early as 1904, but their efforts were irregular and undertaken only when resources permitted. The inspectors, usually called "mounted guards", operated out of El Paso, Texas. Though they never totaled more than 75, they patrolled as far west as California trying to restrict the flow of illegal Chinese immigration.

In March 1915, Congress authorized a separate group of mounted guards, often referred to as "mounted inspectors". Most rode on horseback, but a few operated automobiles, motorcycles, and boats. Although these inspectors had broader arrest authority, they still largely pursued Chinese aliens trying to avoid the National Origins Act and Chinese Exclusion Act of 1882. These patrolmen were immigration inspectors, assigned to inspection stations, and could not watch the border at all times. U.S. Army soldiers along the southwest border performed intermittent border patrolling, but this was secondary to "the more serious work of military training". Aliens encountered illegally in the U.S. by the Army were directed to the immigration inspection stations. Texas Rangers were also sporadically assigned to patrol duties by the state, and their efforts were noted as "singularly effective".

The National Origins Act authorized the formation of the U.S. Border Patrol on May 26, 1924. Two days later, the Labor Appropriation Act of 1924 established the Border Patrol as an agency of the U.S. Department of Labor assigned to prevent illegal entries—primarily along the Mexico–United States border, as well as the Canada–U.S. border. The first Border Patrol station began operations in Detroit, Michigan, in June 1924. A second station, in El Paso, Texas, began operations in July 1924. In 1925, coastal patrols began as well. Operations were established along the Gulf Coast in 1927 to ensure that foreign crewmen departed on the same ship on which they arrived. In 1932, the Border Patrol was divided into two offices. Mexican border operations were directed from El Paso, Texas, and Canadian border operations were directed from Detroit, Michigan. The Canadian border operations from Detroit employed more men than the El Paso operations along the Mexican border because of a focus on the prevention of liquor smuggling during prohibition. Franklin Delano Roosevelt's Executive Order 6166 formed the Immigration and Naturalization Service (INS) in 1933 by consolidation of the Bureau of Immigration and the Bureau of Naturalization. Following the outbreak of World War II in Europe, Border Patrol staffing doubled to 1,500 in 1940, and the INS was moved from the Department of Labor to the U.S. Department of Justice. Additional stations were temporarily added along the Gulf Coast, Florida and the Eastern Seaboard during the 1960s after Fidel Castro triumphed in the Cuban Revolution, and that was followed by the Cuban Missile Crisis. The INS was decommissioned in March 2003 when its operations were divided between U.S. Customs and Border Protection (CBP), United States Citizenship and Immigration Services, and U.S. Immigration and Customs Enforcement.

In the wake of the attacks of September 11, 2001, the Border Patrol was placed under the umbrella of the United States Department of Homeland Security, and preventing terrorists and terrorist weapons from entering the United States was added to its mission. The Border Patrol's traditional mission continued: deterring, detecting and apprehending illegal aliens and individuals involved in the illegal drug trade who generally entered the United States at places other than through designated ports of entry. The Border Patrol also erected^[when?] 33 permanent interior checkpoints near the southern border of the United States.

For fiscal year 2019, the nationwide total of Border Patrol agents was 19,648, with 16,731 patrolling the southern border. Agents primarily patrol the Mexico–United States border, where they control drug trafficking and illegal immigration.