The Warmouth is widespread in the eastern United States, ranging from the lower Great Lakes south to Florida, and west to Kansas. In Canada, the species has only been reported in Lake Erie in southwestern Ontario at Rondeau Bay, Long Point Bay and Point Pelee. A warmwater species, the Warmouth prefers siltfree marshes, ponds and lakes with abundant aquatic plant cover and mucky substrates.

The Warmouth is a small sunfish (up to 30 cm long) with an oval, compressed shape. The mouth is large with a projecting lower jaw and a band of tiny teeth on the tongue. The dorsal fin comprises both soft and spiny rays. There are approximately five dark lines radiating from the snout and eye across the cheek. Dark markings on the sides are suggestive of vertical bands. Usually, the colour is yellow or brown above fading to yellow or white on the underside.