Black soldier fly Hermetia illucens larvae meal (HI), one of the seven insect species approved for use in aquafeed, is a widely studied as an alternative for fish diets. This study aimed to evaluate the effects of dietary inclusion of HI on growth performance, plasma biochemistry, nutrient digestibility, intestinal microbiota, and histopathology of European sea bass Dicentrarchus labrax. Four isoprotein (43,5% CP) and isolipid (13,5%) experimental diets, with HI inclusion levels of 0%, 15%, 30%, and 45%, corresponding to 0%, 33%, 67%, and 100% replacement of fishmeal on a weight-to-weight basis (HI0, HI33, HI67, and HI100, respectively), were fed to juvenile European sea bass (body-weight 85,11 ± 0,87g, total length 19,03± 0,88 cm) for 285 days. The fish were randomly distributed in 12 cylindrical- fiberglass tanks (volume: 800 L), with an initial stocking density of 3,4 kg/m³ (40 fish per tank, in triplicate per diet). They were hand-fed to apparent satiation with one of the four experimental diets twice daily at 8:00 and 15:00, six days a week. The feeding trial was conducted in a RAS system. Temperature and oxygen of the water were measured daily with an average respectively 20,61°C and 5,26 ± 0,23 mg/L. The highest final body weight was recorded in groups fed on HI67 diet. Overall, the average weight gain across the experimental period for fish in all diet groups followed a similar trend, with no significant differences. The effect of HI on specific growth rate (SGR, %/day) was significantly higher (p>0,05) in fish fed HI67 compared to other experimental diets. The feed conversion ratio (FCR) also varied significantly across treatments, with the Duncan test revealing a significant difference between HI67 and the other diets. The best FCR (1,83 ± 0,085), was achieved with the HI67 diet, with no adverse effects on intestinal histomorphology. These findings suggest that Hermetia illucens larvae meal can be a valuable alternative protein source in diets of the European sea bass.