Biological invasion is a major concern for numerous reasons, including the ability of invasive species to disrupt and alter subsidy flow between ecosystems. Sandy beaches, characterized by low primary productivity, are dependent on marine-derived subsidies. Stranded patches of kelp, other algae, and seagrasses, referred to as “wrack”, can support diverse invertebrate beach assemblages that rely on these resources for survival. These assemblages utilize wrack patches differentially in response to a suite of morphological and physiochemical factors that ultimately influence species abundance and composition. To examine changes in beach invertebrate assemblages driven by the replacement of native Macrocystis pyrifera by the invasive alga, Sargassum horneri, I surveyed three beaches in San Diego County, California, USA and conducted a field-based wrack-manipulation experiment to examine differences in invertebrate colonization. I also assessed the resource quality of S. horneri through lab-based consumption and growth experiments using a common organism in beach wrack, Megalorchestia benedicti. There was an influx of S. horneri wrack during the alga’s annual spring senescent phase, and through surveys, I documented ≈ 20% of total macroalgal wrack as S. horneri. Within patches of S. horneri, there were similar numerical and biomass densities of invertebrates in comparison to patches of M. pyrifera, although diversity was consistently lower in S. horneri. M. benedicti found in S. horneri were smaller than those in M. pyrifera, suggesting that S. horneri is an inferior habitat for native assemblages. In the lab, M. benedicti exhibited lower consumption and growth rates with S. horneri than with M. pyrifera. Additionally, as S. horneri continues to expand in range, replacing M. pyrifera as sea temperatures rise, the inflow of native macroalgal subsidies will decline, resulting in subsidy disruption. Furthermore, because wrack composed of S. horneri is only present in the spring, replacement of M. pyrifera, a year-round source of wrack, would potentially leave wrack-reliant invertebrates with few resources during the remainder of the year. Beach invertebrates are experiencing regional declines and local extirpations in response to other anthropogenic stressors, further emphasizing the need to address the effects of S. horneri invasion in disrupting sandy beach assemblages.