The Osam (Bulgarian: Осъм [ˈɔsɐm]) is a river in northern Bulgaria, a right tributary of the Danube.

Its length including the main stem Cherni (Black) Osam is 314 km, making it the fifth longest river in Bulgaria.

The river proper, formed by the confluence of the Cherni and Beli (White) Osam, is 278 km.

[1] The main stem the Cherni Osam is 36 km long and takes its source at an altitude of 1,880 m on the southwestern foothills of the summit of Levski (2,166 m) in the Balkan Mountains and flows north.

Downstream of Aleksandrovo the river enters the Danubian Plain, forming an asymmetrical valley with steeper right and gentle left slopes.

Further north in the fore-Balkan section, the catchment area of the Osam is well forested and grassed and high-stemmed woods gradually give way to low-stemmed ones.

That area is known for its numerous limestone caves, including Raychova Dupka which is the second deepest cave in Bulgaria with a depth of -377 m.[7] Along the left bank of the Cherni Osam south of the village of Oreshak is located the Troyan Monastery, among the largest and most important spiritual sites in the country.