The United States Coast Guard Auxiliary (USCGAUX, CGAux, or USCG Aux) is the uniformed, civilian volunteer component of the United States Coast Guard. Congress established the unit on 23 June 1939, as the United States Coast Guard Reserve. On February 19, 1941, the entity was renamed the United States Coast Guard Auxiliary. The Auxiliary's purpose is to bolster all USCG undertakings both at sea and in the sky, with the exception of tasks necessitating "direct" law enforcement or military actions. As of 2022, the U.S. Coast Guard Auxiliary boasted around 21,000 members.

The Auxiliary collectively dedicates over 4.5 million service hours annually and has accomplished close to 500,000 missions in aid of the Coast Guard. Annually, members of the Auxiliary are instrumental in saving around 500 lives, aiding 15,000 mariners in distress, performing over 150,000 recreational vessel safety checks, and imparting boating safety knowledge to over half a million learners. Collectively, the Coast Guard Auxiliary's efforts save taxpayers several hundred million dollars every year.

USDT 1939–1941
DoN 1941–1945
USDT 1945–1967
USDOT 1967–2003
DHS 2003–present

The advent of the single-operator motorboat, followed by the introduction of the outboard engine in the early 20th century, led to a surge in recreational boating on U.S. federal waters. By the time 1939 rolled around, over 300,000 personal watercraft were in use. In the preceding year, the Coast Guard had been alerted with 14,000 assistance requests and had addressed 8,600 situations where boaters were "in peril."

The U.S. Congress enacted the Coast Guard Reserve Act in 1938, establishing a volunteer reserve unit for the United States Coast Guard. Their primary duties included promoting maritime safety, enhancing boating proficiency among Americans, assisting in legal awareness and adherence, and supporting the active-duty members of the Coast Guard. Boat owners were grouped into flotillas distributed across Coast Guard districts throughout the U.S. These flotillas undertook safety and security patrols and played a role in enforcing the Federal Boating and Espionage Acts of 1940. Commandant Russell Waesche and Commodore Malcolm Stuart Boylan are credited as the founders.

In 1941, just two years after its creation, Congress passed legislation to reorganize the Coast Guard Reserve. From that point onward, the Coast Guard would maintain two reserve entities. The original volunteer group was rebranded as the Coast Guard Auxiliary. Concurrently, a new Coast Guard Reserve was established, endowed with military and law enforcement duties.

During World War II, the Coast Guard recognized a distinct class of reservist known as the "Temporary Reservist." Many of these individuals came from the ranks of the Auxiliary. These Temporary Reservists were uniformed and armed but served without pay, akin to a home guard. Additionally, Coast Guard Headquarters set forth directives permitting certain Auxiliarists and their vessels to bear arms. In 1941 the Coast Guard, Coast Guard Reserve, and Coast Guard Auxiliary were transferred from the United States Treasury Department to the United States Department of the Navy and in 1942 the Coast Guard Auxiliary was authorized to wear military uniforms.

During World War II, Auxiliarists played a pivotal role in assisting the Coast Guard with the recruitment and training of active-duty members. In 1942, due to the escalating threat posed by German U-boats, the U.S. Navy sought the acquisition of "as many civilian craft as feasible that were capable of undertaking voyages at sea under fair weather conditions for a duration of at least 48 hours." A significant portion of vessels, both owned and navigated by Auxiliarists and crewed by Coast Guard reservists, constituted the primary American coastal defense against submarines during the initial stages of World War II, forming what was known as the "Corsair Fleet."