8-week feeding trial was conducted for juvenile rainbow trout (Oncorhynchus mykiss) to determine the optimal amount of Tenebrio molitor meal (TM) that could replace fishmeal in feeds without reducing growth performance, hematological indices and non-specific immune responses. Five isonitrogenous and isolipidic diets were formulated with TM replacing 0% (Control), 25% (TM25), 50% (TM50), 75% (TM75) and 100% (TM100) fishmeal respectively. Each diet was randomly divided into triplicate groups with 20 juveniles (initial weight 10±0.5 g). At the end of the experiment, the mean of the final weight of the fish fed diets with control (41.7±0.7) and diet containing 25% TM powder (43.1±2.1) was significantly higher than other treatments (p<0.05). The lowest feed conversion ratio was observed in control (0.76±0.1) which had a significant difference compared to TM75 (0.93±0.06) (p<0.05). Weight gain (WG) was affected by dietary TM inclusion and decreased with increasing TM. Red blood cells was not affected by treatment, while hemoglobin content and hematocrit decreased with increasing TM inclusion levels (p<0.05). Plasma complement activity, the IgM level and cortisol were not affected by dietary TM. Dietary treatments had no significant effects on plasma glucose and triglyceride of fish compared to the control (p<0.05). Plasma total protein, albumin and globulin were significantly affected by dietary TM (p<0.05). Plasma cortisol levels and alternative complement (ACH50) pathway activity did not show any significant difference among treatments (p>0.05). According to a broken-line model, the recommended substituted ratio of TM for fish meal was estimated to be 25% for rainbow trout obtaining maximum weight gain.