The United States Capitol Police (USCP) is a federal law enforcement agency in the United States with nationwide jurisdiction charged with protecting the United States Congress within the District of Columbia and throughout the United States and its territories. It answers to the Capitol Police Board and is the only full-service federal law enforcement agency appointed by the legislative branch of the federal government of the United States.

The United States Capitol Police has the primary responsibility for protecting life and property, preventing, detecting, and investigating criminal acts, and enforcing traffic regulations throughout a complex of congressional buildings, parks, and thoroughfares. The Capitol Police has primary jurisdiction within buildings and grounds of the United States Capitol Complex. It also has concurrent jurisdiction with other law enforcement agencies, including the United States Park Police and the Metropolitan Police Department of the District of Columbia, in an area of approximately 200 blocks around the complex. Officers also have jurisdiction throughout the District of Columbia to take enforcement action when they observe or are made aware of crimes of violence while on official duties.

Additionally, they are charged with the protection of members of Congress, officers of Congress, and their families throughout the entire United States, its territories and possessions, and the District of Columbia. While performing protective functions, the Capitol Police have jurisdiction throughout the entire United States. It is informally considered as the sister agency of the United States Secret Service, which itself is responsible for the protection of the United States president and their Cabinet.

In the early 1980s, the protective mandates and jurisdictions of the U.S. Capitol Police were substantially expanded to allow them to protect legislators away from their normal jurisdictions in response to the growing risk and threats faced by legislators and the growing institutionalization of Congress.

U.S. Code, Title 2, Chapter 29 defines the powers and duties of the U.S. Capitol Police. The U.S. Capitol Police has the authority to enforce the laws of the United States in any area of the United States and has the power to effect warrantless arrests for any offense against the United States committed in their presence, or for any felony cognizable under the laws of the United States upon probable cause, in the performance of their protective duties. Qualified members of the U.S. Capitol Police may also travel outside of the United States when performing security advisory and liaison functions for congressional travel.

The primary jurisdiction of the United States Capitol Police centers on the United States Capitol building in Washington, D.C., the adjacent congressional (House and Senate) offices, and the Library of Congress buildings. This primary jurisdiction is about 270 acres (0.42 sq mi; 1.1 km²), with about 58 acres (0.091 sq mi; 0.23 km²) being the Capitol grounds themselves.

The U.S. Capitol Police has also concurrent jurisdiction with the District of Columbia's Metropolitan Police Department (MPD), the U.S. Park Police, the U.S. Secret Service Uniformed Division, and others federal agencies to enforce District of Columbia laws, based upon cooperative agreements with MPD and the Police Coordination Act covering local cooperation with 32 federal law enforcement agencies.

The U.S. Capitol Police also have extended jurisdiction over parts of Northeast, Northwest, and Southwest Washington D.C. The USCP provides protection detail to House and Senate leaders, other Members of Congress depending on individual risk analysis, Members' state and district offices (with the help of local police), and "off-campus" events such as presidential nominating conventions.