Parasite prevalence in predatory mammals is influenced by numerous factors including diet, sex, season, and habitat. We examined the effect of such factors on the prevalence of gastrointestinal helminths in North American river otters (Lontra canadensis) in Yellowstone Lake, Yellowstone National Park. Otters in this ecosystem have recently experienced a decline in their main prey, Yellowstone cutthroat trout (Oncorhynchus clarkii bouvieri), and have, in turn, increased consumption of alternative foods. Helminths were found in 13.2% of otter fecal samples. The dominant parasite was a Diphyllobothrium sp. (Diphyllobothrium ditremum or Diphyllobothrium dendriticum), a cestode acquired from cutthroat trout. Truttaedacnitis truttae and Contracaecum spp. nematodes were incidental parasites in otter feces, and acanthocephalan eggs were found in 1 sample. The prevalence of trout remains and a Diphyllobothrium sp. in otter feces was higher during the cutthroat trout spawning season. A Diphyllobothrium sp. was more prevalent in the feces of female otters. There was no relationship between annual declines in the frequency of trout in otter feces and prevalence of parasites. Helminth prevalence and species richness in Yellowstone otters was low and likely reflects low predator densities and few intermediate hosts. This is the first study reporting the helminth fauna of river otters in the Greater Yellowstone Area and confirms the otter as a definitive host for Diphyllobothrium sp. in this region.