The longest river flowing exclusively through Romania is the Olt.
The Olt is approximately 615 kilometres long, from its source in the Hășmaș Mountains to its mouth on the Danube near Turnu Măgurele. Unlike the Prut, Siret, and Mureș, which cross or form part of Romania’s international river systems, the Olt remains entirely within Romanian territory.
It travels south through several regions and counties, including Harghita, Covasna, Brașov, Sibiu, Vâlcea, Olt, and Teleorman. Important cities along its course include Miercurea Ciuc, Sfântu Gheorghe, Râmnicu Vâlcea, and Slatina.
The wording “flowing exclusively through Romania” matters. Romania’s longest rivers by total or nationally measured length can include international rivers such as the Mureș and Prut. The Olt is the canonical answer specifically because its full course lies inside Romania, even though it ultimately joins the international Danube system.