The bony-tongue fish genus Arapaima Müller has been considered monotypic since 1868, with A. gigas being the only recognized species. Review of species-level taxonomy of Arapaima has revealed that Arapaima agassizii Valenciennes (in Cuvier and Valenciennes, 1847) should be considered a valid species. The holotype was destroyed in World War II, but the species can be recognized based on the original description, which included detailed osteological illustrations. At least nine characters distinguish it from all other Arapaima: 1) dentary teeth 44 (counted on one ramus only, vs. 21–37 in other Arapaima); 2) maxillary teeth 43 (vs. 21–38 in other Arapaima); 3) orbit diameter 1.5% standard length (SL, vs. 1.5–2.8, relatively larger in all other Arapaima at similar SL); 4) interorbital width 4.1% SL (vs. 5.3–6.5 in other Arapaima); 5) parietals with pronounced posterior projections that are pointed and curve slightly toward midline (vs. absent in other Arapaima); 6) caudal fin widely separated from dorsal and anal fins by long caudal peduncle, 9.7% SL (vs. much shorter peduncle, 3.2–5.5 in others); 7) anal fin with only 26 rays (vs. 30–40 in others), with distinctly shorter basal length than dorsal-fin base; 8) dorsal and anal fins extremely low in profile; dorsal-fin base divided by longest dorsal ray about 7 (vs. 3.1–5.5 in others); and 9) first pectoral-fin ray with proximal tip similar in form to subsequent pectoral-fin rays (vs. first pectoral-fin ray noticeably enlarged relative to subsequent rays). Arapaima agassizii still is known only from the holotype, which was collected in 1817–20 somewhere in lowlands of the Brazilian Amazon. It thus is important to locate this taxon to determine its distribution and conservation status.