The river was first sighted by a European in 1525, by the explorer García Jofre de Loaísa, and was called Río San Idelfonso. Simón de Alcazaba y Sotomayor's 1535 expedition was the first to name the river "Río Gallegos". The area was not settled by Europeans until much later, with one of the earliest recorded indications of habitation in 1885, when the Argentine government wished to better express its sovereignty over southern Patagonia — a naval base was created which increased the development of the town. Between 1912 and 1920, the government encouraged settlers from the Falkland Islands (Spanish: Islas Malvinas) and southern Chile with preferential farming conditions. Some 3,000 arrived and boosted the town. As sheep-farming increased, Río Gallegos became the principal port for exporting sheep and their products. The Pioneers Museum is a preserved old Patagonian house exhibiting the life of the early settlers.
Governor Ramón Lista decided to move the Territorial Capital from Puerto Santa Cruz to Río Gallegos in 1888; official ratification of this decision came on 19 May 1904. The Cathedral of Our Lady of Luján was consecrated in 1900. The Territory of Santa Cruz was declared a Province in 1957, with Mario Cástulo Paradelo as its first Governor.
In the 1982 Falklands War, the city airport was base of Argentine Air Force Mirage III interceptors and A-4 Skyhawks strike aircraft.
Néstor Kirchner, President of Argentina from 2003 to 2007, was mayor of Río Gallegos between 1987 and 1991.
Today, Río Gallegos is an important city of the Argentine far south, with military bases and an international airport. Flights between Chile and Mount Pleasant airbase at the Falkland Islands stop at Río Gallegos once a