Actinomycosis, presenting as chronic proliferative pyogranulomatous pleuritis associated with sulfur granules, was diagnosed in a mature male river otter (Lontra canadensis) captured near Victoria, British Columbia, Canada, in April 2010. The river otter died during sedation for clinical examination and was necropsied immediately. Bacteria of the genus Actinomyces are commensals of the mucous membranes in a wide range of animals and people that act as opportunistic pathogens. This is the first report of actinomycosis in a river otter.