<p>In this study, we tested the potential of photo-identification as an identification method for bull trout using I³S Classic, a program that utilizes natural markings to identify individuals. To determine the ability to identify individual bull trout from spotting patterns, we used photographs of bull trout captured in 2017–2019 by the Idaho Department of Fish and Game at Fourth of July Creek, Idaho. During weir operations, 132 unique bull trout were sampled of which 34 fish were recaptured (<i>n</i> = 166 total captures). Using pictures of these fish, we created “spotting fingerprints” in the I³S system for each fish and compared “fingerprints” to identify recaptures. In a blind test, three readers used I³S to determine which fish were recaptures and which were not. The three readers correctly assigned a mean of 92.7% (SD = 1.6%) of the bull trout as recaptured fish or fish captured for the first time. For the recaptured fish, the three readers were able to correctly identify 72% (SD = 6%) of the fish; however, the correct identification rate increased to 87% (SD = 7%) after removing fish with poor image quality that were misidentified by all readers. The readers correctly identified 100% of the fish that were not recaptured, i.e., the false-positive rate was zero. If standardization of photograph quality and spot selection can be established, photo-identification could be a complementary tool that could be used to monitor the population dynamics of bull trout.</p>