Shallow water subtidal marine communities along the western Antarctic Peninsula are characterized by dense beds of macroalgae and strikingly dense assemblages of associated amphipods. However, direct grazing by amphipods on the dominant macroalgae is unlikely as most of these algae elaborate secondary metabolites known to be herbivore feeding deterrents. What resources, then, support this vast macroalgal-associated amphipod assemblage? We addressed this question by analyzing the gut contents, fatty acids, and stable isotopic ratios of 15 different amphipod species associated with the macrophyte community. The δ15N and δ13C stable isotope values revealed that most of the abundant species of amphipods are primary consumers whose ultimate carbon source is derived from some combination of brown macroalgae, epiphytic diatoms, and endo/epiphytic filamentous algae. Gut contents revealed that a large percentage of the amphipod diets are comprised of diatoms and macroalgal tissues, both filamentous and multiseriate. Fatty acid analysis corroborated our conclusions based on stable isotope and gut content data, demonstrating the importance of diatoms to assimilated material, but also highlighting the rich diversity of diets within the macroalgal-associated amphipod assemblage. Our findings suggest that amphipods routinely clean their host macrophytes of potentially harmful epiphytes, including both diatoms and emergent filaments from brown algal endophytes. Some prominent species of amphipods may also derive a small percentage of their carbon from palatable and, in one case, unpalatable, chemically defended red algae. These results, combined with previous studies showing that the amphipods gain refuge from predators by associating with unpalatable macroalgae, support the hypothesis that amphipods along the western Antarctic Peninsula are living in mutualism with their macrophyte hosts rather than consuming the host directly.