Hydrolycus armatus is a species of dogtooth characin found in freshwater of tropical South America.

This predatory fish occasionally makes its way into the aquarium trade, but it requires a very large tank.

[5][6] This species of fish is found in the Amazon, Orinoco and Essequibo basins in tropical South America.

[8] The species is locally common; in a major study of a Venezuelan floodplain river, 1⁄3 of the collected fish were H. armatus, and the species was particularly common in creeks and lagoons.

In adults the base of the tail and anal fin is pale yellowish, and the distal part is blackish, contrasting with a narrow white edge at the very tip (unique among Hydrolycus species).