The effects of dietary single-cell proteins (SCP) derived from the filamentous fungus Paecilomyces variotii (PV) on growth, plasma biochemistry, and gut health were assessed in gilthead sea bream. Fish, with an initial average weight of 120.3 ± 0.02 g, were fed four experimental diets for 104 days, incorporating different levels of PV SCP meal (0% CTRL, 5% SCP5, 7.5% SCP7.5, and 10% SCP10) as a partial replacement for fishmeal (FM) starting from a FM content of 22% in the control diet. At the end of the trial fish were subjected to a 2-h acute crowding stress (80 kg m3 biomass). At the end of the growth trial, there were no significant differences in final body weight (FBW), specific growth rate (SGR), feed conversion rate (FCR), or feed intake (FI), indicating that growth and feed utilization were not adversely affected by the inclusion of SCP. Instead, the diet containing PV upregulated the expression of genes related to nutrient transport, such as peptide transporter 1 (pept1), fatty acid-binding protein (fabp2), sodium/glucose cotransporter 1 (slc5a1), and aquaporin 8 (aquap8), suggesting an enhanced capacity for nutrient uptake. Varying levels of dietary SCP led to an increased abundance of potentially beneficial lactic acid bacteria, as well as the genus Paenibacillus, known for its chitinase activity, which may enhance nutrient availability through chitin degradation. Crowding stress (T2) significantly altered most plasma parameters, reflecting a negative impact on the fish's metabolic functions. Notably, fish fed SCP-based diets exhibited substantially lower plasma levels of aspartate aminotransferase (AST), lactate dehydrogenase (LDH), and creatine kinase (CK), suggesting a mitigating effect on stress-induced tissue damage. This protective role is further supported by the upregulation of glutathione reductase (gr) expression, indicating enhanced antioxidant defenses capable of counteracting oxidative damage in muscle and other tissues. In conclusion, these findings confirm the potential of the filamentous fungus Paecilomyces variotii as a valuable protein source for gilthead sea bream, not only as a partial substitute for conventional proteins in aquafeeds but also as a functional ingredient with health-promoting properties