The Rienz (German pronunciation: [ˈriːɛnts] ; Italian: Rienza [riˈɛntsa]) is a river in South Tyrol, Italy. Its source is located at 2,180 m of altitude, in the Dolomites mountains, south of Toblach: near Toblach it enters in the Puster Valley, and, after 90 km (56 mi), it meets the Eisack river in the city of Brixen, at 550 m of altitude.

The Rienz flows through the following municipalities (source to mouth): Toblach, Niederdorf, Welsberg-Taisten, Olang, Rasen-Antholz, Bruneck, St. Lorenzen, Kiens, Vintl, Mühlbach, Rodeneck and Brixen.

The most significant affluents are:

• Ahr, forming the Ahrntal and responsible of 1/3 of the total discharge.
• Antholzer Bach.
• Gran Ega (Val Badia).
• Gsieser Bach.
• Pragser Bach.
• Pfunderer Bach.
• Wielenbach.

The maximum discharge of the Rienz is around 60 m/s.