Nomorhamphus sagittarius can be diagnosed from all congeners by the following combination of characters: lower jaw elongated; one to seven teeth along dorsal surface of extended portion of lower jaw (absent in some specimens); fins and ventral surface of lower jaw orange; absence of black fin pigmentation; base of the pectoral fin with a distinct black spot; a lanceolate, dorsally slightly curved spiculus in the male andropodium, its middle segments in contact with the distal tip of the third anal-fin ray; seven to ten segments proximal to spinae (mode eight); segments three to seven (mode three to five) of second anal-fin ray in males with a dorsal and a ventral row of ‘subsegments’ forming small squares and rectangles of different sizes, so that these segments seem to be subdivided; second anal-fin ray with an elongated fourth or fifth segment (mode fifth) in some males (78.6% of the adult male type specimens); and third anal-fin ray slightly constricted longitudinally, giving the appearance of two distinct rays, distal part of this ray is slightly curved ventrally to contact spiculus (Ref. 97329).