The Tet Offensive (Tet Mau Than), or officially, Tổng Công Kích/Tổng Khởi Nghĩa - General Offensive, General Uprising, was a three-phase military campaign conducted between 30 January and 23 September 1968, by the combined forces of the National Front for the Liberation of South Vietnam (NLF or Viet Cong) and the People's Army of Vietnam (PAVN) during the Vietnam War. The purpose of the operations, which were unprecedented in this conflict in their magnitude and ferocity, was to strike military and civilian command and control centers throughout the Republic of Vietnam (South Vietnam) and to spark a general uprising among the population that would then topple the Saigon government, thus ending the war in a single blow.

The operations are referred to in the West as the Tet Offensive because they were timed to begin during the early morning hours of 31 January, Tết Nguyên Đán, the lunar new year holiday. For reasons that are still not completely understood, a wave of attacks began on the preceding morning in the I and II Corps Tactical Zones. This early attack did not, however, cause undue alarm or lead to widespread allied defensive measures. When the main NLF-PAVN operation began the next morning, the offensive was countrywide in scope and well coordinated, with more than 80,000 communist troops striking 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 national capital. Dougan & Weiss, p. 8. The offensive was the largest military operation yet conducted by either side up to that point in the war.

The initial Communist attacks stunned allied forces and took them by surprise, but most were quickly contained and beaten back, inflicting massive casualties on the NLF. The exceptions were the fighting that erupted in the old imperial capital of Huế, where intense fighting lasted for a month, and the continuing struggle around the U.S. combat base at Khe Sanh, where fighting continued for two more months. Although the offensive was a military disaster for communist forces, it had a profound effect on the American administration and shocked the American public, which had been led to believe by its political and military leaders that the communists were, due to previous defeats, incapable of launching such a massive effort.

The majority of Western historians have concluded that the offensive ended in June, which easily located it within framework of U.S. political and military decisions that altered the American commitment to the war. In fact, the General Offensive continued, according to plan, through two more distinct phases. The second phase began on 5 May and continued until the end of the month. The third began on 17 August and only ended on 23 September.