The Army of the Republic of Vietnam (ARVN; Vietnamese: Lục quân Việt Nam Cộng hòa; French: Armée de la république du Viêt Nam) composed the ground forces of the South Vietnamese military from its inception in 1955 to the Fall of Saigon on 30 April 1975. Its predecessor was the ground forces of the Vietnamese National Army, established on 8 December 1950, representing Vietnam to fight in the First Indochina War against the communist Viet Minh rebels. At the ARVN's peak, an estimated 1 in 9 citizens of South Vietnam were enlisted, composed of Regular Forces and the more voluntary Regional Forces and the Popular Force militias. It is estimated to have suffered 1,394,000 casualties (killed and wounded) during the Vietnam War.

The ARVN began as a post-colonial army that was trained by and closely affiliated with the United States and had engaged in conflict since its inception. Several changes occurred throughout its lifetime, initially from a 'blocking-force' to a more modern conventional force using helicopter deployment in combat. During the American intervention in Vietnam, the ARVN was reduced to playing a defensive role with an incomplete modernisation, and transformed again following Vietnamization, it was upgeared, expanded, and reconstructed to fulfill the role of the departing American forces. By 1974, it had become much more effective with foremost counterinsurgency expert and Nixon adviser Robert Thompson noting that Regular Forces were very well-trained and second only to the American and Israeli forces in the Free World and with General Creighton Abrams remarking that 70% of units were on par with the United States Army.

However, the withdrawal of American forces by Vietnamization meant the armed forces could not effectively fulfill all of the aims of the program and had become completely dependent on U.S. equipment since it was meant to fulfill the departing role of the United States. Unique in serving a dual military-civilian administrative purpose, in direct competition with the Viet Cong, the ARVN had also become a component of political power and suffered from continual issues of political loyalty appointments, corruption in leadership, factional infighting, and occasional open internal conflict.

After the fall of Saigon to North Vietnam's People's Army of Vietnam (PAVN), the ARVN was dissolved. While some high-ranking officers had fled the country to the United States or elsewhere, thousands of former ARVN officers were sent to re-education camps by the communist government of the unified Socialist Republic of Vietnam. Five ARVN generals died by suicide to avoid capture.

On 8 March 1949, after signing the Élysée Accords, the State of Vietnam was recognized by France as an independent country ruled by the Vietnamese Emperor Bảo Đại, and the Vietnamese National Army (VNA) was soon created on 8 December 1950. The VNA fought in joint operations with the French Union's French Far East Expeditionary Corps against the communist Viet Minh forces led by Ho Chi Minh. The VNA fought in a wide range of campaigns including the Battle of Nà Sản (1952), Operation Atlas (1953) and the Battle of Dien Bien Phu (1954).

Benefiting from French assistance, the VNA quickly became a modern army modeled after the Expeditionary Corps. It included infantry, artillery, signals, armored cavalry, airborne, airforce, navy and a national military academy. By 1953, troopers as well as officers were all Vietnamese, the latter having been trained in Ecoles des Cadres such as Da Lat, including Chief of Staff General Nguyễn Văn Hinh who was a French Union airforce veteran.

After the 1954 Geneva agreements, French Indochina ceased to exist and by 1956 all French Union troops had withdrawn from Vietnam, Laos, and Cambodia. In 1955, by the order of Prime Minister Diệm, the VNA crushed the armed forces of the Bình Xuyên.

On 26 October 1955, the military was reorganized by the President Ngô Đình Diệm who declared the republic in the State of Vietnam. The air force was established as a separate service known as the Republic of Vietnam Air Force (RVNAF). Early on, the focus of the army was the guerrilla fighters of the Viet Cong (VC), formed to oppose the Diệm administration. The United States, under President John F. Kennedy sent advisors and a great deal of financial support to aid the ARVN in combating the insurgents. A major campaign, developed by Ngô Đình Nhu and later resurrected under another name was the "Strategic Hamlet Program" which was regarded as unsuccessful by Western media because it was "inhumane" to move villagers from the countryside to fortified villages. ARVN leaders and Diệm were criticized by the foreign press when the troops were used to crush armed anti-government religious groups like the Cao Đài and Hòa Hảo as well as to raid Buddhist temples, which according to Diệm, were harboring VC guerrillas. The most notorious of these attacks occurred on the night of August 21, 1963, during the Xá Lợi Pagoda raids conducted by the ARVN Special Forces, which caused a death toll estimated to range into the hundreds.