A 77-day feeding trial was designed in order to assess the effect of partial or total replacement of soybean meal by hazelnut meal (HNM) on feed intake, growth performance, nutrient utilization and body composition of rainbow trout (initial mean body weight: 80.0 +/- 3.5 g) in seawater conditions. A control and three experimental diets were formulated in which soybean meal level (25.5%) in the control diet was replaced by hazelnut meal at 39.2 (HN 39), 78.4 (FIN 78) and 100% (RN 100) to provide 44% crude protein and 20% crude lipid diets. Diets were fed to 125 rainbow trout to apparent satiation by hand twice daily at 09.00 and 16.00 hours under natural photoperiod conditions. At the end of the feeding trial, fish of all groups almost tripled their body weight and no significant difference (P>0.05) was revealed in final weight or specific growth rate between treatments. However, feed conversion ratio (FCR) of the HN100 group was significantly higher than other treatments (P<0.05). There was a slightly decreasing trend in protein efficiency ratio (PER) and net protein utilization (NPU) with increasing level of dietary hazelnut meal, but it was only significant in groups of fish fed HN100. All groups of fish displayed similar carcass and muscle compositions. According to the results of the present study, it can be stated that hazelnut meal can replace soybean meal at up to 200 g kg(-1) in grow-out diets of rainbow trout without any detrimental effects on growth performance.