The sides of the fish are mottled and lack the horizontal stripes of other Triplefins. The first dorsal fin is as high (or higher) than the second dorsal fin. The length of the fish is usually about 9 cm.

Can be found in tidal marshes and estuaries from the intertidal zone to about twelve metres deep. Often associated with red algae. Sometimes it can be found in the lower reaches of rivers and streams.

Feeds upon amphipods (sandhoppers), crabs and shrimps.

Deverall, K. R., 1979. The distribution and abundance of Forsterygion nigripenne (CUV. AND VAL. 1836) in Otago Harbour. Unpub BSc (Hons) Thesis University of Otago. http://fishbase.mnhn.fr/summary/Forsterygion-nigripenne.html (accessed 07/05/13)