Rhadinoloricaria macromystax is a species of freshwater ray-finned fish belonging to the family Loricariidae, the suckermouth armored catfishes, and the subfamily Loricariinae, the mailed catfishes. The genus was monotypic until 2020, when a new species, R. stewarti, was described.
Rhadinoloricaria macromystax is endemic to Peru where it occurs in the upper Amazon. It is known to occur over sandy substrates. The species reaches a standard length of . This detrivorous, benthic species lives in white and clear water rivers with soft substrates made up of sand or lime.
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