The Republic of Vietnam Navy (RVNN; Vietnamese: Hải quân Việt Nam Cộng hòa - HQVNCH; French: Marine de la république du Vietnam) was the naval branch of the South Vietnamese military, the official armed forces of the former Republic of Vietnam (or South Vietnam) from 1955 to 1975. The early fleet consisted of boats from France; after 1955, and the transfer of the armed forces to Vietnamese control, the fleet was supplied from the United States. With American assistance, in 1972 the RVNN became the largest Southeast Asian navy and, by some estimates, the fourth largest navy in the world, just behind the Soviet Union, the United States and the People's Republic of China, with 42,000 personnel, 672 amphibious ships and craft, 20 mine warfare vessels, 450 patrol craft, 56 service craft, and 242 junks. Other sources state that RVNN was the ninth largest navy in the world. The Republic of Vietnam Navy was responsible for the protection of the country's national waters, islands, and interests of its maritime economy, as well as for the co-ordination of maritime police, customs service and the maritime border defence force.

The RVNN disbanded in 1975 with the collapse of South Vietnam, and North Vietnam's victory in the Vietnam War. Most of its fleet was captured in port, but a small fleet of vessels, led by Captain Đỗ Kiếm and Richard L. Armitage of the Defense Attaché Office, Saigon, escaped to Thailand and surrendered themselves to American naval forces there. Some of these RVNN vessels were scuttled upon reaching the open sea, while others continued their service with the Philippine Navy.

RVNN commander Captain Hồ Tấn Quyền, was a loyal supporter of President Ngô Đình Diệm. In order to prevent him supporting Diệm in the 1963 South Vietnamese coup, he was executed by fellow RVNN officers on the morning of 1 November 1963.

During the 1965 South Vietnamese coup, rebel forces surrounded the RVNN headquarters at the Saigon Naval Shipyard, apparently in an attempt to capture RVNN commander Chung Tấn Cang. However, this was unsuccessful and Cang moved the fleet to Nhà Bè Base to prevent the rebels from seizing the ships.

In early 1969, President Richard M. Nixon formally adopted the policy of "Vietnamization". The naval part, called ACTOV ("Accelerated Turnover to the Vietnamese"), involved the phased transfer to Vietnam of the U.S. river and coastal fleet, as well as operational command over various operations. In mid-1969, the RVNN took sole responsibility for river assault operations when the U.S. Mobile Riverine Force stood down and transferred 64 riverine assault craft to the RVNN. On 10 October 1969, 80 Patrol Boat, Rivers (PBR) were transferred to the RVNN at the Saigon Naval Shipyard, the PBRs were divided into four River Patrol Groups (RPGs) as part of Task Force 212.

On 19 January 1974, four RVNN ships fought a battle with four ships of the Chinese People's Liberation Army Navy over ownership of the Paracel Islands, 200 nautical miles (370 km) due east of Đà Nẵng. The RVNN ship Nhựt Tảo (HQ-10) was sunk, Lý Thường Kiệt (HQ-16) was heavily damaged, and both Trần Khánh Dư (HQ-4) and Trần Bình Trọng (HQ-5) suffered light damage. The Chinese captured and occupied the islands. On 30 January 1974 the RVNN mounted Operation Tran Hung Dao 48 to station troops on unoccupied islands to assert Vietnam's sovereignty over the Spratly archipelago.

In the spring of 1975, North Vietnamese forces occupied all of northern and central South Vietnam, and finally Saigon fell on 30 April 1975. American military attaché Richard Armitage and RVNN Captain Kiem Do had secretly planned and then carried out the evacuation of a flotilla of 35 RVNN and other vessels, with 30,000 sailors, their families, and other civilians aboard to Côn Sơn Island, where they met USS Kirk, who escorted the flotilla to U.S. Naval Base Subic Bay, 1,000 miles (1,609.3 km) away.

Due to the poor condition and overcrowding of many of the vessels in the flotilla, the five-mile long convoy was forced to travel at the speed of the slowest boat, 6 knots, or over 6 days. By the time the convoy approached Philippine waters, the Government of the Philippines had acknowledged the legitimacy of the North Vietnamese government, so the ships were technically PRG property. Armitage decided to repatriate the vessels, with U.S. junior officers, NCOs, and Armitage himself taking command of the 32 vessels to sail them into Subic under the U.S. flag where they were later given to the Philippine Navy. LSM Lam Giang (HQ-402), fuel barge HQ-474, and gunboat Kéo Ngựa (HQ-604) transferred their cargo of refugees and their crews to other ships, and were scuttled off Côn Sơn after it was determined they would not survive the journey to Subic. RVNS My Tho (HQ-800) remains one of the last former RVNN vessels still in commission today as BRP Sierra Madre (LT-57).