The Miocene fossil fish †Pentaceros sakhalinicus , (Perciformes, Pentacerotidae) is redescribed based on the holotype and numerous additional materials collected on Sakhalin Island (western Pacific, Russian Far East) at the end of the 20th century. The general appearance and body proportions, as well as the presence of multiple (more than three) anal-fin spines, make the Miocene taxon similar to members of the family Pentacerotidae, whereas generalized morphological characters such as the presence of three supraneurals and the absence of rugosity of the superficial head bones differentiate this species greatly from any known pentacerotid genus. Thus, the new genus †Portentosoceros is erected for this fish, which was considered the only known fossil member of the family. Due to the fact that this fish lacks the main synapomorphic characters of the Pentacerotidae, †Portentosoceros gen. nov. is left incertae sedis within the Percoidei, though probably related to Recent armorheads. The observed variability in the number of analfin spines (from four to six) in †Portentosoceros sakhalinicus is extremely unusual.