Black soldier fly ( Hermetia illucens ) larvae meal (BSFLM) is a novel fish feed ingredient with a high crude protein content (~40% dry matter, DM). However, BSFLM also contains chitin, which might impair nutrient digestibility. To investigate the effect of chitin on nutrient digestibility and chitinase activity in fish, BSFLM with different chitin contents were tested as feed ingredients for Nile tilapia ( Oreochromis niloticus ) and rainbow trout ( Oncorhynchus mykiss ). BSFLM was sieved into three size fractions obtaining different chitin contents: 0–200 μm (fine), 200–400 μm (medium), >400 μm (coarse), 1.8, 2.7 and 15.4% chitin on a DM basis, respectively. Four isoenergetic and isonitrogenous diets were formulated for each species including an experimental reference diet and three diets replacing 25% of the experimental reference diet matrix with one of the three BSFLM size fractions. Dietary inclusion of the coarse BSFLM fraction resulted in significantly lower apparent digestibility of DM, crude protein, nitrogen‐free extract and chitin when compared with the other treatment groups in both fish species, which supports the hypothesis that chitin might act as an anti‐nutrient. However, it was also found that both species could digest chitin but its digestibility decreased with a higher dietary chitin inclusion level. Furthermore, β‐N‐acetylglucosaminidase activity (exochitinase) increased with higher dietary chitin inclusion in the proximal and distal intestine of both fish species suggesting that exochitinase production can be upregulated depending on the dietary chitin intake. Overall, the results corroborate that chitin could act as a nutrient source as well as an anti‐nutrient in both Nile tilapia and rainbow trout.