The United States Coast Guard (USCG) is a maritime, military, multi-mission service unique among the military branches for having a maritime law enforcement mission (with jurisdiction in both domestic and international waters) and a federal regulatory agency mission as part of its mission set. It operates under the Department of Homeland Security during peacetime, and can be transferred to the Department of the Navy by the President or Congress during time of war.

Founded by Alexander Hamilton as the Revenue Cutter Service on 4 August 1790, it lays claim to being the United States' oldest continuous seagoing service. As of August 2009, the Coast Guard had approximately 42,000 men and women on active duty, 7,500 reservists, 29,000 auxiliarists, and 7,700 full-time civilian employees.

The Coast Guard's legal authority differs from the other four armed services. It operates simultaneously under Title 10 of the US Code and its other organic authorities, e.g., Titles 6, 14, 19, 33, 46, etc. Because of its legal authority, the Coast Guard can conduct military operations under the DOD or directly for the President in accordance with Title 14 USC 1-3. The Coast Guard's enduring roles are Maritime Safety, Maritime Security, and Maritime Stewardship. To carry out those roles the Coast Guard has eleven statutory missions as defined in 6 U.S.C. § 468