Kosový potok, Brook in Karlovy Vary Region, Czechia.

Kosový potok is a brook in the Karlovy Vary Region that winds through forested valleys for approximately 46 kilometers. It originates in higher elevations and descends gradually in multiple curves until it joins the Mže River.

The stream's German name Amselbach, with upper-course variants Wonschabach, Wunschelbach and Altbach, is recorded in German sources; the Czech name Kosový potok is used alongside Kosí potok. Its valley from Dolní Kramolín was declared the Kosový potok Nature Park in 1995. The brook flows past settlements such as Valy, Klimentov and Třebel before joining the Mže.

The brook is known in Czech as Kosový potok or Kosí potok, and in German as Amselbach, "blackbird brook"; the upper course also carried the names Wonschabach, Wunschelbach and Altbach. From Dolní Kramolín downstream it runs through the valley declared Přírodní park Kosí potok in 1995, and there it forms the natural border between the Kaiserwald and the Oberpfälzer Wald.

The brook is best accessed on foot via walking trails that run along the valley through different forest sections. The trails remain accessible year-round, though winter conditions can make walking more challenging.

The German name Amselbach means "blackbird brook", and the upper course also carried the names Wonschabach, Wunschelbach and Altbach. The stream rises at 721.5 m near the stone marking the former centre of Europe, two kilometres southwest of it on the Czech-German border.