The Biebrza (Lithuanian: Bebra,[1] Belarusian: Bobra, German: Bober) is a river in northeastern Poland, a tributary of the Narew River (near Wizna), with a length of 164 kilometers (102 mi) and a basin area of 7,092 km2 (7,067 in Poland).

Historically, the borderland regions of Mazovia and Lithuania, the area retains much of its cultural diversity today.

The basin of the Biebrza River is inhabited not only by hundreds of rare and endangered sorts of birds, but also by people representing diverse cultures, languages, and religions.

Although most of the population of the region speaks standard Polish, some people in the upper river basin (municipalities of Lipsk, Dąbrowa Białostocka and partly Sztabin) speak a local dialect of Belarusian (called by them prosty jazyk 'simple language').

On the north bank of the upper Biebrza there are also a few villages where so-called "old believers" live, who speak an archaic dialect of Russian.