The United States Secret Service (USSS or Secret Service) is a federal law enforcement agency under the Department of Homeland Security tasked with conducting criminal investigations and providing protection to American political leaders, their families, and visiting heads of state or government. The Secret Service was, until 2003, part of the Department of the Treasury, due to their initial mandate of combating counterfeiting of U.S. currency. The agency has protected U.S. presidents and presidential candidates since 1901.

The Secret Service is mandated by Congress with two distinct and critical national security missions: protecting the nation's leaders and safeguarding the financial and critical infrastructure of the United States.

The Secret Service is tasked with ensuring the safety of the president, the vice president, the president-elect, the vice president-elect, and their immediate families; former presidents, their spouses and their children under the age of 16; those in the presidential line of succession, major presidential and vice-presidential candidates and their spouses; and visiting foreign heads of state and heads of government. By custom, it also provides protection to the secretary of the treasury and secretary of homeland security, as well as other people as directed by the president (usually the White House chief of staff and national security advisor, among others). Former Secret Service director Kimberly Cheatle told the congressional oversight committee that as of July 22, 2024 the secret service has a total of 36 protectees. By federal statute, the president and vice president may not refuse this protection. The Secret Service also provides physical security for the White House Complex; the neighboring Treasury Department building; the vice president's residence; the principal private residences of the president, vice president and former presidents; and all foreign diplomatic missions in Washington, D.C. The protective mission includes protective operations to coordinate manpower and logistics with state and local law enforcement in the US, protective advances to conduct site and venue assessments for protectees, and protective intelligence to investigate all manners of threats made against protectees. The Secret Service is the lead agency in charge of the planning, coordination, and implementation of security operations for events designated as National Special Security Events (NSSE). As part of the service's mission of preventing an incident before it occurs, the agency relies on advance work and threat assessments developed by its Intelligence Division to identify potential risks to protectees.

The Secret Service is tasked with safeguarding the payment and financial systems of the United States from a wide range of financial and cyber-based crimes. Financial investigations include counterfeit U.S. currency, bank and financial institution fraud, mail fraud, wire fraud, illicit financing operations, and major conspiracies. Cyber investigations include cybercrime, network intrusions, identity theft, access device fraud, credit card fraud, and intellectual property crimes. The Secret Service is also a member of the FBI's Joint Terrorism Task Force (JTTF) which investigates and combats terrorism on a national and international scale. Also, the Secret Service investigates missing and exploited children and is a partner of the National Center for Missing & Exploited Children (NCMEC).

The Secret Service's initial responsibility was to investigate the counterfeiting of U.S. currency, which was rampant following the American Civil War. The agency then evolved into the United States' first domestic intelligence and counterintelligence agency. Many of the agency's missions were later taken over by subsequent agencies such as the Federal Bureau of Investigation (FBI), Central Intelligence Agency (CIA), Drug Enforcement Administration (DEA), Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF), and IRS Criminal Investigation Division (IRS-CI).

The Secret Service is also tasked with investigating reports of the existence of specimens of the extremely rare 1933 double eagle gold coin, as only a single example of the fourteen known survivors from this mintage year is authorized to be owned or sold.

The Secret Service combines the two responsibilities into a dual objective. The two core missions of protection and investigation synergize with each other, providing benefits to special agents during the course of their careers. Skills developed during the course of investigations which are also used in an agent's protective duties include but are not limited to:

Protection of the nation's highest elected leaders and other government officials is one of the primary missions of the Secret Service. After the 1901 assassination of President William McKinley, Congress also directed the Secret Service to protect the president of the United States. The Secret Service investigates thousands of incidents each year of individuals threatening the president of the United States.