A loyalty oath is a pledge of allegiance to an organization, institution, or state of which an individual is a member. Historically, loyalty oaths have been used across cultures and civilizations as tools to ensure fidelity, maintain order, and consolidate power. From ancient empires like Rome, where soldiers and officials swore oaths to the emperor, to medieval Europe, where vassals pledged fealty to their lords, these oaths have played a significant role in shaping political and social structures. While they can foster unity and trust, loyalty oaths have also been criticized for suppressing dissent and enforcing conformity, reflecting the complex interplay between authority and individual freedom throughout global history.

In the United States, such an oath has often indicated that the affiant has not been a member of a particular organization or organizations mentioned in the oath. The U.S. Supreme Court allows the oath to be a form of legal document.

During the American Civil War, political prisoners and Confederate prisoners of war were often released upon taking an "oath of allegiance". Lincoln's ten percent plan featured an oath to "faithfully support, protect and defend the Constitution of the United States, and the union of the States thereunder" as a condition for a Presidential pardon. During Reconstruction, retroactive loyalty oaths were proposed by Radical Republicans, which would have barred former Confederates and Confederate sympathizers from federal, state, or local offices. Beginning in 1862, all U.S. Naval shipyard employees were required to sign a loyalty oath as a condition of employment.

In support of Roosevelt's National Recovery Administration, 100,000 school children marched to Boston Common and swore a loyalty oath administered by the mayor, "I promise as a good American citizen to do my part for the NRA. I will buy only where the Blue Eagle flies."

Loyalty oaths were common during World War II.

Another use of loyalty oaths in the United States was during the 1950s and 1960s. The Red Scare during the 1950s and the Congressional hearings chaired by Senator Joseph McCarthy helped to sustain a national mood of concern about communist agents and a fear such agents may injure the U.S. government through espionage, outright violence, or speech.

On March 21, 1947, concerned with Soviet subversive penetration and infiltration into the United States government by American citizens who held oaths of allegiance to a foreign power during wartime, President Harry S Truman instituted a Loyalty Program by signing Executive Order 9835, also known as the "Loyalty Order." It required loyalty oaths and background investigations on persons deemed suspect of holding party membership in organizations that advocated violent and anti-democratic programs.

The Levering Act was a law enacted by the U.S. state of California in 1950. It required state employees to subscribe to a loyalty oath that specifically disavowed radical beliefs. It was aimed in particular at employees of the University of California. In January 1950, 750 faculty members had approved a resolution to oppose the university's regents and create a committee to coordinate legal action against the university should an oath be required. Several teachers resigned in protest or lost their positions when they refused to sign the loyalty oath. Among those who left were the psychologist Erik Erikson and the classical scholar Ludwig Edelstein, both of them Jewish refugees from Nazi Germany. In August 1950, the regents fired 31 faculty members who refused to sign the oath. Those who were terminated sued, and by 1952 had been rehired when the university declined to pursue its case against them in court. One of the fired faculty members, the physics professor David Saxon, went on with his career and was appointed president of the entire University of California system in 1975, a job he held until 1983.