The United States Department of Homeland Security (DHS) is a Cabinet department of the Federal Government of the United States, with the responsibility of protecting the territory of the United States from terrorist attacks and responding to natural disasters. The department was established on November 25, 2002, by the Homeland Security Act of 2002. It was intended to combine U.S. executive branch organizations related to "homeland security" into a single Cabinet agency. DHS officially began operations on January 24, 2003, but most of the department's component agencies were not transferred into the new Department until March 1 of that year.

The Department of Homeland Security works in civilian territory to protect the United States within, at, and outside its borders. Its goal is to prepare for, prevent, and respond to domestic emergencies, particularly terrorism. On March 1, 2003, the DHS absorbed United States Immigration and Naturalization Service (INS), and assumed its duties.

With over 200,000 employees, DHS is the third largest cabinet department in the U.S. federal government after the Department of Defense and Department of Veterans Affairs. Homeland security policy is coordinated at the White House by the Homeland Security Council. Other agencies with significant homeland security responsibilities include the Department of Health and Human Services, the Department of Justice, and the Department of Energy.