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Ninety‐six juvenile specimens (37–54 mm standard length;L_S) of the rarely collected Upward‐Mouth SpikefishAtrophacanthus japonicus(Triacanthodidae) were obtained from the stomachs of three Yellowfin TunaThunnus albacarescollected off Guam in the Mariana Islands in the central Pacific Ocean. These specimens extend the range ofA. japonicuseastward into Oceania. We review the systematic characters of the monotypic genusAtrophacanthusand present colour photographs of freshly collected specimens. The diet of the juvenile specimens ofA. japonicusconsisted of thecosome pteropods and foraminiferans. We present a range map ofA. japonicusbased on all known specimens and show that specimen size is related to whether specimens were collected in the pelagic zone or on the bottom. Our results support that, compared to all other Triacanthodidae,A. japonicushas an unusually extended pelagic larval and juvenile period, up to 54 mmL_S, before settling to the bottom as adults. Lastly, we provide a multilocus phylogeny addressing the phylogenetic placement ofAtrophacanthusbased on eight of 11 triacanthodid genera and six genetic markers. Our results reveal thatAtrophacanthusis the sister group ofMacrorhamphosodesand they provide new insights about the evolutionary history of the family.