The Reserve Officers' Training Corps (ROTC /ˌrɒtsiː/ or /ˌɑːr oʊ tiː ˌsiː/) is a group of college- and university-based officer-training programs for training commissioned officers of the United States Armed Forces.

While ROTC graduate officers serve in all branches of the U.S. military, the U.S. Marine Corps, the U.S. Space Force, and the U.S. Coast Guard do not have their own respective ROTC programs; rather, graduates of Naval ROTC programs have the option to serve as officers in the Marine Corps contingent on meeting Marine Corps requirements. Graduates of Air Force ROTC also have the option to be commissioned in the Space Force as a Space Operations Officer. Although the Coast Guard does not operate an ROTC program, the Coast Guard Auxiliary University Programs (AUP) provides students with training and opportunities to participate in Coast Guard activities, however, participation in the AUP does not guarantee a commission.

In 2020, ROTC graduates constituted 70 percent of newly commissioned active-duty U.S. Army officers, 83 percent of newly commissioned U.S. Marine Corps officers (through NROTC), 61 percent of newly commissioned U.S. Navy officers and 63 percent of newly commissioned U.S. Air Force officers, for a combined 56 percent of all active-duty officers in the Department of Defense commissioned that year. Under ROTC, a student may receive a competitive, merit-based scholarship covering all or part of college tuition, textbooks and lab fees, in return for an active-duty service obligation after graduation (or completion of a graduate degree under an approved education delay). ROTC students attend college like other students, but also receive basic military training and officer training for their chosen branch of service through the ROTC unit at or nearby the college. The students participate in regular drills during the school year and off-campus training opportunities during the summer.

Army ROTC units are organized as brigades, battalions and companies. Air Force ROTC units are detachments with the students organized into wings, groups, squadrons and flights. Army and Air Force ROTC students are referred to as cadets. Naval ROTC units are organized as battalions and also include NROTC students under "Marine Option" who will eventually be commissioned as officers in the Marine Corps. Marine NROTC students may be formed in a separate company when the program includes sufficient numbers. All Naval ROTC students are referred to as midshipmen. Some of the summer training that is offered to cadets in the Army ROTC program are: Airborne, Air Assault, Mountain Warfare, WHINSEC and other related schools. In addition to their mandatory 20 day Field Training (FT) at Maxwell AFB, Alabama, typically between their sophomore and junior year. Air Force ROTC cadets are also eligible for Airborne training under the tutelage of the Army at Fort Benning, Georgia. Naval ROTC midshipmen will participate in summer cruise programs every summer, either afloat or ashore, similar to their U.S. Naval Academy midshipmen counterparts.

The concept of training reserve military officers in civilian colleges and universities in United States was created by the founder of Norwich University, Alden Partridge, who was a former United States Military Academy instructor. Partridge, who founded Norwich in Northfield, Vermont in 1819 as the "American Literary, Scientific and Military Academy," promoted the idea of "citizen soldiers," men trained to act in a military capacity when their nation required but capable of fulfilling standard civilian functions in peacetime. The Morrill Act of 1862 established land-grant colleges. Part of the federal government's requirement for these schools was that they include military tactics as part of their curriculum. Another root of the modern ROTC program comes from the "Plattsburg Idea". In 1915, Major General Leonard Wood instituted the Citizen's Military Training Camps (not to be confused with the later CMTC of the interwar period), the first series of training camps to make officers out of civilians. For the first time in history, an attempt was made to provide a condensed course of training and commissioning competent reserve line officers after only a summer of military training.

In 1916, the provision to formally establish ROTC was advocated to Congress by a delegation from Ohio including William Oxley Thompson, President of the Ohio State University. On February 7, 1916, Ralph D. Mershon, a graduate of Ohio State, testified before the committee as a professional engineer. Present to testify as an advocate of a Reserve Engineers Corps, he expanded his remarks to argue in favor of the "Ohio Plan". Mershon noted:

Congress agreed, and the ROTC provision was included in the final version of the National Defense Act of 1916. The first ROTC unit was at Harvard in 1916.

Over 5,000 men arrived at Plattsburgh, New York, in May 1917 for the first of the officer training camps. By the end of 1917, over 17,000 men had been trained. By the eve of its entry into World War I, the U.S. had a prepared corps of officers including one of the earliest Plattsburgh graduates, Theodore Roosevelt Jr.