This study assessed the effects of dietary fat level and source on growth performance, and seawater acclimation of the Caspian brown trout. Tuna fish oil and a blend of canola and soybean oils (85: 15) with different levels of fat (LF: 10% and HF: 20%) were made to produce four different diets. The diets were labeled accordingly: LFFO (low fat fish oil), LFVO (low fat vegetal oil), HFFO (high fat fish oil) and HFVO (high fat vegetable oil). Each diet was fed to Caspian brown trout parr (similar to 10 g) for 60 days. Both diets with vegetable oil base and high dietary fat (i.e. 20%) improved the growth performance, only the fishes fed a LFFO diet gained significantly less weight than the ones fed the other diets (P < 0.05). The seawater acclimation was assessed by a gill Na+K+-ATPase immunolocalization (chloride cells number, size and area ratio) and by measuring the concentrations of serum chloride, sodium, magnesium, cortisol and glucose before transferring the fishes to seawater as well as 5 days later. The fish fed with the test diets had no significant difference in chloride concentration but magnesium concentration significantly increased in all treatments after the seawater transfer. Sodium concentration increased significantly (P < 0.05) only in fish fed the LFFO diet after the seawater transfer. Nor was there any difference in the number of filament chloride cells, but the fish fed this LFFO diet showed low salt secretion ability in SW due to the smaller size and area ratio of the filament chloride cells. The cortisol increased significantly only with the HFVO treatment after transfer to seawater (P < 0.05). This experiment provides evidence that, due to higher energy supply, favorable fatty acids for energy production and facilitating better acclimation of the fish to Caspian Sea water, a high dietary fat level with vegetable oil (HFVO) meet adequately the essential nutritional requirements of the Caspian brown trout parr.