Reintroducing fish to previously occupied habitats appears promising for recovery of extirpated fish populations in cold water systems. Uncertainty still exists surrounding the ecological effects of reintroductions however, particularly when they involve historically sympatric taxa. We initiated a study to estimate any potential impacts to rainbow/steelhead trout (Oncorhynchus mykiss) that may arise from reintroducing coho salmon (O. kisutch) in Taneum Creek, Washington, following their extirpation approximately 100 years ago. Prior to reintroducing coho salmon into Taneum Creek, we conducted a formal risk assessment to predict potential impacts to rainbow trout that might result from restoring coho salmon natural production in this stream. Following the assessment, adult coho salmon were released to spawn naturally in experimental reaches in Taneum Creek during a five year period, 2008–2012. Rainbow trout abundance, average size, condition, and growth were not reduced in our experimental reaches relative to control locations following the reintroduction of coho salmon; a result predicted from our ecological risk assessment. Our findings validate the utility of the ecological risk assessment for predicting and reducing undesirable effects of reintroductions involving historically sympatric salmonids.