An ongoing research program has investigated the ichthyofaunal structure and dynamics of the oceanic northern Gulf of Mexico (GoM), from the surface to 1500 m depth. Prior to 2011 there was no inventory of the GoM bathypelagial (>1000 m depth) and that of the mesopelagial (200-1000 m) was largely limited to the eastern GoM. Extensive sampling and analysis since that time has revealed an exceptionally speciose oceanic fish assemblage with inherent pelagic endemicity. Of the 794 fish species identified to date, 180 are new records for the GoM, including one newly described and 21 putative undescribed species. This increases the total fish species number for the entire GoM marine ecosystem by over 10%. Over half of all known fish species in the GoM use the oceanic habitat for part or all of their lives. The GoM now ranks among the most-speciose oceanic ichthyofaunal assemblages known in the World Ocean. This ranking likely results from: 1) the ecotonal nature of the oceanic GoM (low-latitude input, winter cooling); 2) high (non-limiting) oxygen at depth; 3) substantial use of pelagic habitat by juvenile deep-demersal and coastal fishes; 4) sampling intensity (the GoM stands as the world’s most-studied bathypelagic system); and 5) the integration of morphological and genetic methodology used to increase taxonomic accuracy and precision. Despite extensive sampling, the species accumulation curve has not reached asymptote; more species will likely be recorded with more sampling.