Gymnocephalus is a genus of ray-finned fishes from the family Percidae, which includes the perches, pike-perches and darters.

They have the common name "ruffe" and resemble the true perches in the genus Perca, but are usually smaller and have a different pattern.

The species within the genus Gymnocephalus have a number of characters in common including that their dorsal fins are not completely separate, they have enlarged canals extending from the lateral line on their heads, the preorbital bone covers the maxillary bone, presence of setiform or bristle-like teeth, having very few or no vomerine and palatine teeth and the possession of three paired bones in the neck, known as extrascapulars, in their lateral line system, of which, two are simple tubes.

It has traditionally been placed in the subfamily Percinae alongside the true perch of the genus Perca.

[4] The 5th edition of Fishes of the World treats Gymnocephalus as the only genus in the monotypic subfamily Acerinae,[6] although Gill's Gymnocephalinae is referred to in some sources.