This study investigated the effects of astaxanthin, which is used to improve the muscle color of fish, on the nutritional composition of rainbow trout (Oncorhynchus mykiss). The trout were fed extruded pellets containing astaxanthin (pigmented rainbow trout) or without astaxanthin (non-pigmented rainbow trout). No significant differences in the contents of crude protein and crude ash between the two muscles of the two groups were observed. However, the crude fat composition of the muscle of pigmented rainbow trout was about two times higher than that of control muscle. In the muscle of pigmented rainbow trout, the contents of docosahexaenoic acid (DHA, 22:6n-3) and eicosapentaenoic acid (EPA, 20:5n-3), which are functional polyunsaturated fatty acids, were 1.6 fold higher than in non-pigmented muscle. In addition, the contents of minerals such as Zn and Fe, vitamins (B group, C, and E), free amino acids, carotenoids and astaxanthin were increased in the muscle of rainbow trout fed pellets supplemented with astaxanthin. Specifically, the content of the bioactive compound astaxanthin, was increased six times in the pigmented muscle, as compared to the control muscle. The results suggest that a diet supplemented with astaxanthin improves the nutritional composition of rainbow trout muscle as well as improving the muscle color.