Cryonotothenioids dominate the fish fauna of the Antarctic continental shelf and represent one of the best-studied examples of adaptive radiation in marine teleosts. Pogonophryne, the barbeled plunderfishes, is the most species rich cryonotothenioid genus and is hypothesized to be among the fastest diversifying cryonotothenioid lineages. However, the credibility of this hypothesis of exceptional diversification rests upon the delimitation of Pogonophryne species boundaries. Most of the 29 currently recognized species of Pogonophryne are distinguished solely by variation in ornamentation of the mental barbel that projects from the lower jaw, a structure previously shown to vary widely within a single species. Ten species of Pogonophryne were described in the past decade and are collectively known from only 21 specimens. Here, we integrate phylogenetic, population genetic, and coalescent analyses of genome-wide sequence data with investigation of variation in body shape, morphological meristic traits, and mental barbel morphology to delimit species of Pogonophryne. We propose to dramatically reduce the recognized species diversity of Pogonophryne by synonymizing 24 of currently recognized species with five species of Pogonophryne described in a period from 1914 to 1981. In addition, we find both genomic and phenotypic evidence for a new species of Pogonophryne from specimens collected in the Ross Sea. We discuss how this new perspective on Pogonophryne species diversity reshapes our understanding of the mechanisms generating diversity in the cryonotothenioid adaptive radiation.