The Dwarf Flathead Gudgeon is usually dark brown above and lighter brown to pale orange ventrally. Two dark lines radiate posteriorly from the eye. The first dorsal fin has two stripes of black with a broad orange margin. The caudal fin is pale orange basally becoming grey posteriorly with two to four dark bands formed by spots on the fin rays.
This species grows to 5 cm in length, but is more often seen at 3.5 cm to 4 cm in length.
The Dwarf Flathead Gudgeon is endemic to Australia. It occurs in freshwater and brackish to full strength seawater in the upper reaches of estuaries. It shows considerable variation over its wide distribution from southern Queensland through New South Wales, Victoria and eastern South Australia. The species appears to be less commonly in parts of the Murray River drainage.
For many years the species was recorded in the literature as Philypnodon sp. In 2006, Australian Museum scientists Dr Doug Hoese and Ms Sally Reader named the species P. macrostomus. The species name macrostomus comes from the Greek words macro and stoma, meaning 'large' and 'mouth' respectively.