Diagnosed from all congeners by having the following characters: extremely slender dorsal most lateralis sensory cavity on preopercle, strongly angled ventrolateral margin of head where third infraorbital meets anterior limb of preopercle such that ventral surface of head is almost flat, and enlarged thickened sheath on anterior third of dorsal-fin base that hides anterior dorsal-fin rays when adpressed. Differs also from all other Arapaima except A. agassizii by having extremely long fourth infraorbital. Distinguished from A. arapaima and all other examined non-type specimens by having notably slender body; from A. mapae and A. agassizii by having relatively deep caudal peduncle (6.0% SL, vs. ,4.0 in both of latter two taxa); from A. gigas by having dentary teeth in a single row (vs. dentary teeth in 2-2.5 irregular rows); and from A. agassizii by having 28 teeth on maxilla (vs. 43) and on single ramus of dentary (30-32, vs. 44) (Ref. 94273).