The Tet Offensive was a major escalation and one of the largest military campaigns of the Vietnam War. It was launched on January 30, 1968 by forces of the Viet Cong (VC) and North Vietnamese People's Army of Vietnam (PAVN) 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, with the offense chosen during a holiday period 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, beat back the attacks, and inflicted heavy casualties on PAVN/VC forces. The popular uprising anticipated by Hanoi never happened. During the Battle of Huế, intense fighting lasted for a month, resulting in the destruction of the city. During their occupation, the PAVN/VC executed thousands of people in the Massacre at Huế. Around the U.S. 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 due to 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 see that the current war strategy required re-evaluation. The offensive had a strong effect on the U.S. government and shocked the U.S. 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 ramping up of draft calls. Subsequently, the Johnson Administration sought negotiations to end the war, which were derailed in a secret agreement between then-former Vice President Richard Nixon, who planned to run as the Republican candidate in the 1968 United States presidential election, and South Vietnamese President Nguyễn Văn Thiệu.

The term "Tet offensive" usually refers to the January–February 1968 offensive, but it can also include the so-called "Mini-Tet" offensive that took place in May and the Phase III offensive in August, or the 21 weeks of unusually intense combat which followed the initial attacks in January.