Megalagrion leptodemas (Perkins, 1899)  is endemic to the Hawaiian island of O'ahu, and is considered one of the rarest and most vulnerable of all endemic Megalagrion species in Hawaii. Megalagrion leptodemas was found historically in the mountain ranges of Koolau and Waianae, and is currently restricted to scattered sites in four drainages in the Koolau Range. Its limited habitat and small scattered populations may affect long-term stability. The species is susceptible to the effects of habitat loss and introduced species. Research should focus on habitat management and protection, and control of invasive species.

It is one of the smaller Hawaiian damselflies, about 36-41 mm in length with a wingspan of 39-42 mm. Males are slender, with brilliant red coloration on the head, thorax, and abdomen, and black markings on the top of abdominal segments five through seven. Females are similar in color but have pale green to grayish-olive markings on the body, and the top of their abdomen is mostly black.

Larvae of this species reach 200 mm in length. The three flattened, oval, leaf-like gills at the tip of the abdomen have prominent branching veins, and are darker-colored at the base and paler at the tips. The predaceous aquatic nymphs inhabit standing pools in intermittent mid-elevation streams, and slow sections of perennial upland streams. Adults are quick fliers but do not disperse far from the nymphal habitat, patrolling short lengths of the stream and laying eggs in the slow reaches of streams and in stream pools.

Recent surveys have indicated that this species is confined to widely scattered subpopulations in four drainages in the Koolau Mountains of Oahu. One of these catchments has been the site of recent freeway construction.  There probably are less than 1,000 individuals remaining at present. IUCN Red List accessed as Critically Endangered B1ab(iii,v).