Old River is a nature reserve in Central Balkan National Park, Bulgaria.

It was declared a 1974,7 hectare reserve with Order No. 200 of the Nature Conservation Committee under the Ministerial Council of March 19, 1981, in order to preserve the unique combination of coniferous, deciduous and mixed forests.

The reserve is best known for its plant variety. It features a warm-eyed hawthorn, hornbeam and water hornbeam, as well as beech, fir and plain spruce.

More than 45 species of plants are present on the territory of the Reserve, which are present in the Bulgarian Red Book. 20 of these species are not found outside Bulgaria, and three of them can be seen only within the Reserve.

It is located in the central Stara Planina mountain over the town of Karlovo, it covers the catchment of the Old River.