The Koštěnický potok (German: Neumühlbach) is a stream in the Czech Republic, a right tributary of the Lužnice River.

It flows through the South Bohemian Region, and a short section forms the Austrian-Czech state border.

[2] The longest tributaries of the Koštěnický potok are:[3] The stream flows through the municipal territories of Kunžak, Člunek, Kačlehy, Číměř, Nová Bystřice, Stráž nad Nežárkou, Staňkov, Chlum u Třeboně, Hamr and Majdalena.

A section of the stream between Stráž nad Nežárkou and Haugschlag with a length of about 1.5 km (0.9 mi) forms the Austrian-Czech state border.

The reason of protection is the preserved ecosystem of the stream and its floodplain with the occurrence of endangered species of plants and animals, especially the green snaketail.