In the Salish Sea, red sea urchins (Mesocentrotus franciscanus) are influential herbivores in subtidal ecosystems. They consume a variety of macroalgae but often prefer Nereocystis luetkeana . These urchins may play a vital role in regulating the macroalgae composition of the ecosystem, however Sargassum muticum has become an influential presence in the ecosystem, possibly competing with local macroalgal species growing around the Salish Sea. It has not yet been established how the presence of S. muticum affects the feeding behavior of M. franciscanus and if they will consume this common, but non-native, species. In this study I investigated this relationship and found that the urchins will feed on N. luetkeana over S. muticum but will consume S. muticum when there is no other choice. If S. muticum continues to spread throughout the Salish Sea, the feeding preferences of M. franciscanus will likely compound the competition between S. muticum and N. luetkeana adversely affecting the native species.