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Water depth can affect the size spectra (SS) and biomass of free-living nematodes, but these relationships are largely unknown for the Gulf of Mexico (GoM). Here, we examined the SS and biomass of meiofaunal nematode assemblages at 27 sites along a water-depth gradient from 44 to 3466 m in the Perdido belt, northwestern GoM. Nematodes were identified to species level, measured (length and width), and assigned to shape morphotypes and trophic groups. Nematode SS were defined by a median length of 951 μm (range: 223–4289 μm) and a median width of 33 μm (range: 12–109 μm). SS revealed a dominant long/slender morphotype (e.g., Paramicrolaimus sp., Filoncholaimus sp., and Ledovitia sp.), better adapted to the very fine sediments of the deep sea, whereas stout/short nematodes (mostly Desmoscolex sp., Greeffiella sp., and Quadricoma sp.) were less abundant. Water-depth gradient significantly influenced mean individual (IB) and total biomasses likely because reduced availability of organic carbon with depth. The two deepest sites had particularly high mean IB suggesting local sources of organic carbon in the abyssal region related to the Perdido belt oil field. The most abundant trophic group was microbial feeder sucker (56%), followed by microbial feeder scraper (23%), and predator ingester (20%). The mean IB of predator nematodes was significantly larger than that of nematode preys. However, the logarithm of ratio IBpredator/IBprey was rather low (0.34) and in agreement with the empiric consumer-resource body-size relationship calculated for marine invertebrates. Our study provides novel insights about the structure of benthic nematode assemblages in the western Gulf of Mexico across a wide depth gradient. Inglés