The Tet Offensive was a major escalation and one of the largest military campaigns of the Vietnam War. The North Vietnamese People's Army of Vietnam (PAVN) and its Viet Cong (VC) launched a surprise attack on 30 and 31 January 1968 against the forces of the South Vietnamese Army of the Republic of Vietnam (ARVN), the United States Armed Forces and their allies. It was a campaign of surprise attacks against military and civilian command and control centers throughout South Vietnam. The name is the truncated version of the Lunar New Year festival name in Vietnamese, Tết Nguyên Đán, a holiday period was chosen as most ARVN personnel were on leave. The purpose of the wide-scale offensive by the Hanoi Politburo was to trigger political instability in a belief that mass armed assault on urban centers would trigger defections and rebellions.

The offensive was launched prematurely in the early morning hours of 30 January in large parts of the I and II Corps Tactical Zones of South Vietnam. This early attack allowed allied forces some time to prepare defensive measures. When the main operation began during the early morning hours of 31 January, the offensive was countrywide; eventually more than 80,000 PAVN/VC troops struck more than 100 towns and cities, including 36 of 44 provincial capitals, five of the six autonomous cities, 72 of 245 district towns and the southern capital. The offensive was the largest military operation conducted by either side up to that point in the war.

Hanoi had launched the offensive in the belief that it would trigger a popular uprising leading to the collapse of the South Vietnamese government. Although the initial attacks stunned the allies, causing them to lose control of several cities temporarily, they quickly regrouped, repelled the attacks and inflicted heavy casualties on PAVN/VC forces. The popular uprising anticipated by Hanoi never materialized. During the Battle of Huế, intense fighting lasted for a month, resulting in the destruction of the city. During its occupation, the PAVN/VC executed thousands of people in the Massacre at Huế. Around the American combat base at Khe Sanh, fighting continued for two more months.

The offensive was a military defeat for North Vietnam, as neither uprisings nor ARVN unit defections occurred in South Vietnam. However, this offensive had far-reaching consequences for its effect on the views of the Vietnam War by the American public and the world broadly. General Westmoreland reported that defeating the PAVN/VC would require 200,000 more American soldiers and activation of the reserves, prompting even loyal supporters of the war to admit that the current war strategy required reevaluation. The offensive had a strong effect on the U.S. government and shocked the American public, which had been led to believe by its political and military leaders that the North Vietnamese were being defeated and incapable of launching such an ambitious military operation. American public support for the war declined as a result of the Tet casualties and the escalation of draft calls. Subsequently, the Johnson administration sought negotiations to end the war. Shortly before the 1968 United States presidential election, Republican candidate and former vice president Richard Nixon encouraged South Vietnamese president Nguyễn Văn Thiệu to become publicly uncooperative in the negotiations, casting doubt on Johnson's ability to bring peace.

The term "Tet offensive" usually refers to the January–February 1968 offensive, but it can be extended to cover all of the 21 weeks of unusually intense combat that followed the initial attacks in January (thus including the so-called "Mini-Tet" offensive that took place in May), or even to include the Phase III offensive in August.

Leading up to the Tet Offensive were years of marked political instability and a series of coups after the 1963 South Vietnamese coup. In 1966, the leadership in South Vietnam, represented by the Head of State Nguyễn Văn Thiệu and Prime Minister Nguyễn Cao Kỳ were persuaded to commit to democratic reforms in an effort to stabilize the political situation at a conference in Honolulu. Prior to 1967, the South Vietnamese constituent assembly was in the process of drafting a new constitution and eventual elections. The political situation in South Vietnam, after the 1967 South Vietnamese presidential election, looked increasingly stable. Rivalries between South Vietnam's generals were becoming less chaotic, and Thiệu and Kỳ formed a joint ticket for the election. Despite efforts by North Vietnam to disrupt elections, higher than usual turnouts saw a political turning point towards a more democratic structure and ushered in a period of political stability after a series of coups had characterized the preceding years.

Protests, campaigning and the atmosphere of elections were interpreted by the Politburo of the Communist Party of Vietnam and Lê Duẩn as signs that the population would embrace a 'general uprising' against the government of South Vietnam. The Politburo sought to exploit perceived instability and maintain political weakness in South Vietnam.

During late 1967, the question whether the U.S. strategy of attrition was working in South Vietnam weighed heavily on the minds of the American public and the administration of President Lyndon B. Johnson. General William C. Westmoreland, the commander of the Military Assistance Command, Vietnam (MACV), believed that if a "crossover point" could be reached by which the number of communist troops killed or captured during military operations exceeded those recruited or replaced, the Americans would win the war. There was a discrepancy, however, between the order of battle estimates of the MACV and the Central Intelligence Agency (CIA) concerning the strength of VC guerrilla forces within South Vietnam. In September, members of the MACV intelligence services and the CIA met to prepare a Special National Intelligence Estimate that would be used by the administration to gauge U.S. success in the conflict.