The present study aimed to evaluate physicochemical properties and structural variations of grape seed extract exposed to thermal process during feed production, and the use of grape seed extract in rainbow trout diets as a sustainable organic fish feed supplement. Structural changes of grape seed extract granules were observed under a scanning electron microscope, and analyzed using HPLC/UV and GC/MS. Following thermal process, an increase in the granular size of grape seed was observed and cracks were formed on the granules. In the flavor analysis performed using a GC/MS device, totally nine volatile compounds were identified and three of them were palmitic, linoleic and oleic fatty acids. Thermal processing treatment decreased (+)-catechin amount in grape seed extract from 0.667 to 0.557 mg/L, whereas (-)-epicatechin increased from 0.819 to 1.156 mg/L. As a second step of the study, grape seed extract, a waste product of the wine industry, was supplemented in rainbow trout (Oncorhynchus mykiss) diets at concentrations of 0, 0.5, 1, and 2 parts per thousand and given to experimental fish for a period of 90 days. Feed and protein utilization coefficients as well as growth performance of the experimental fish fed diets containing grape seed extracts differed significantly (P < .05) compared to the control group fed diets without grape seed extract supplementation. Dietary incorporation of grape seeds had no significant effect on the fat, moisture or ash contents in fish fillets (P > .05), however protein ratio significantly increased in the 0.5 parts per thousand grape seed extract group compared to the control (P < .05). Based on these findings and the correlation between dietary grape seed levels and percent growth rates, grape seed extract incorporation of 1.05 parts per thousand seems beneficial for the best growth performance and meat quality in rainbow trout. Further, increased levels of (-)-epicatechin (a polymeric flavan-3-ol), known to participate in many physiological regulatory mechanisms likely have provided higher antioxidant activity after thermal processing. Thus, thermal treatment of grape seed extract during feed production might have increased the diet quality with promising results as a potential nutritional supplement in diets for rainbow trout.