Trematomus nicolai is a near shore benthic notothenioid fish most abundant in the subzero shelf waters of East Antarctica. During recent collecting we obtained the first specimens of this species from West Antarctica (the Bransfield Strait), and we compare these with specimens from the Ross Sea (McMurdo Sound) in East Antarctica. Because T. nicolai has been frequently misidentified as T. tokarevi, we provide several non-meristic characters that separate these species. We employ a radiographic technique for rapid visualization of the cephalic lateral-line canals, an important diagnostic character in trematomids. Compared to those from McMurdo Sound, the Bransfield Strait specimens have lower ranges and mean counts for meristic characters, with significant differences for anal rays, pectoral rays, and vertebrae. Our data suggest a panmictic population, but the Bransfield Strait specimens live in water 3–4°C warmer than McMurdo Sound, and this may contribute to lower meristic counts. The mean buoyancy between the two samples is not significantly different. We examined sequence variation in the ND2 portion of the mitochondrial DNA (mtDNA) genome for evidence of population structure in samples from both areas. We identified 12 mtDNA haplotypes (haplotype diversity [h] = 0.978, nucleotide diversity [π] = 0.458%) and analysis of molecular variance (AMOVA) shows no significant global differentiation. A median-joining network that represents the genealogical relationships among the mitochondrial haplotypes also shows little separation between the samples from West and East Antarctica, and additional tests suggest the T. nicolai population is in equilibrium and of constant size. Trematomus nicolai exemplifies the potential of the Antarctic current regime for circum-Antarctic dispersal of a variety of organisms in the Southern Ocean.