The main objective of this study was to determine the impact of dietary fat on the organic selenium requirement of rainbow trout. There were two variables in this study. The first was organic selenium that was supplemented at 0.1, 0.15 and 0.2 mg · kg^–1. The second variable was the fat inclusion level as low-fat (15%) and high-fat (30%) diets. This led to formulation of six experimental diets according to a 3 × 2 factorial design. Juvenile rainbow trout with an initial weight of 6.99 ± 0.9 g were assigned to 18 tanks with 25 fish each, with three replicates for each diet. The results showed that the tissue oxidation rate increased with increased dietary fat content. The initial and final fish stocking densities were 0.58 and 2.08 kg · m^–3, respectively. Addition of dietary selenium increased glutathione peroxidase activity. Final weight and specific growth rate were not influenced by the fat level of the diets. On the high-fat diet, selenium improved the feed conversion ratio by depression of the oxidation rate. The dietary selenium requirement depended on the dietary fat content and 0.15 mg · kg^–1 was found to be necessary for the high-fat diet. A reduction in oxidation level by inclusion of selenium into the diets was observed and was associated with the higher activity of glutathione peroxidase.