Rosso is the major city of south-western Mauritania and capital of Trarza region. It is situated on the Senegal River at the head of the river zone allowing year-round navigation.

The town is 204 km south of the capital Nouakchott.

Rosso was once the capital of the Emirate of Trarza, a Precolonial Sahrawi State.

Under French colonial rule Senegal and Mauritania were administered as a single entity. When Independence came, the new frontier was drawn along the Senegal River, splitting the small town of Rosso in two. This article refers to Mauritanian Rosso, on the northern bank of the river.

Originally a staging-post for the gum arabic trade, Rosso has grown rapidly since independence.

In 1986, the town was elevated to the status of urban commune. The commune's population was 50,560 in 2013, with 33,518 living in the city of Rosso itself. Having grown from a mere 2,300 in 1960, it has now overtaken Kaédi to become the 3rd largest city in the country.

Southwestern Mauritania is historically a predominantly Wolof-speaking area, but many members of Mauritania's other ethnic groups have moved to the town to escape the severe problems of desertification further north.

Rosso occupies a strategic position at the international ferry-crossing on the main road between Nouakchott and the Senegalese capital of Dakar. Economically the town has benefited, but its fortunes are very dependent on the state of relations between the two countries.