Plectroglyphidodon sagmarius is a small damselfish known only from the waters around Marquesas Islands in the Pacific Ocean, which belong to French Polynesia.
The small endemic damsel is a harmless herbivore.
The colour of adult fish is white with a broad brown bar on the head; 3 brownish-yellow bars on the body; a large black saddle on the caudal peduncle (absent in juveniles); and an orange-yellow spiny portion on the dorsal fin.
Juvenile animals show with a large elliptical black spot at the base and below the rear half of the soft part of the dorsal fin extending to the front part of the caudal peduncle, followed by a pale white spot.
Aquaristic husbandry data are missing, it is also not to be expected that the little fellow will one day be found in European marine aquariums.