The fish intestinal environment is colonised by a complex microbiome community, which plays a critical role in host physiology and health. Antibiotic compounds have previously been reported to induce short and long-term changes in the gut microbiomes of higher vertebrate animals however, the influence of commercially licensed antibiotics on the gut microbiome of farmed fish remains unclear. In this study, we investigated the effect of oxytetracycline, a broad-spectrum antibiotic licensed for use in aquaculture, on gut bacterial communities in rainbow trout (Oncorhynchus mykiss) using high-throughput sequencing. This was evaluated using a 4-week feeding trial where fish were fed diets surface-coated with or without oxytetracycline for 7-days followed by a 14-day withdrawal period. DNA extractions from hind-gut digesta was then subjected to Illumina amplicon-sequencing of the bacterial 16S rRNA gene to profile changes in bacterial communities across time and treatment. Analysis of community composition revealed that oxytetracycline rapidly stimulated changes in the gut microbiome community of treated fish. Within two days of antibiotic treatment, treated groups had lower relative abundances of the Tenericutes phylum, leading to an enrichment of Proteobacteria members. However, following antibiotic treatment, we noted a shift in the bacterial communities of treated fish and increase in alpha diversity. By 14-days withdrawal from oxytetracycline, beta diversity was significantly differentiated between treatment groups and treated fish displayed an enrichment in members of the Cyanobacteria, Proteobacteria and Spirochaetes phyla. This study provides new knowledge on the dynamic changes that occur in response to and following antibiotic treatment in fish microbiome communities.