In the present study, we describe a pathogenic incidence of Shewanella sp. S-1 infection in cultured rainbow trout (Oncorhynchus mykiss, Walbaum, 1792), as evidenced by lens atrophy, inappetence, visual impairment, and growth retardation. Strain S-1 was identified using basic microbiological tests, the BD PhoenixTM rapid diagnostic kit, the MALDI-TOF MS system, and whole-genome analysis. This strain's pathogenicity was assessed based on the histopathological examination of the eyes, lens, muscle, brain, liver, kidney, spleen, and gills. The lens, brain, and eyes of the fish were also examined morphologically. According to genome-based delineation, strain S-1 was determined to belong to the genus Shewanella and was considered a putative novel Shewanella species. While we determined that the lens had atrophied at a level ranging from 30% to 40%, no severe lesion was detected in other organs. Fish that hatched in the same period were bred under uniform feeding and management conditions for a month. Monitoring data revealed that, at the end of this 1-month period, the growth rate and feed conversion rate of the diseased fish were significantly lower than those of the healthy group. Based on microbiological and pathological findings, we concluded that Shewanella sp. S-1 caused lens atrophy in the rainbow trout.