<p>Primary stress response, osmoregulation, and growth were investigated in juvenile lumpfish (mean initial weight (g, ± SEM) was 27.1 ± 1.1—28.0 ± 1.3) reared at three different salinities (15, 20, and 36 ppt) for 28&#xa0;days. Thermal growth coefficient (TGC) was between 4.0 ± 0.12 (15 ppt) and 4.2 ± 0.06 (36 ppt) and did not vary significantly between the three salinity groups. Plasma ions concentrations (Na⁺, K⁺, Cl⁻, and Ca²⁺) were unaffected by salinity and did not provide evidence of osmotic stress in the tested range. Plasma cortisol levels were similar at the beginning and end of the experiments and were similar to, or significantly lower, than values previously reported on undisturbed lumpfish. Overall, present findings indicate that juvenile lumpfish tolerate well low salinity within the tested range which encompasses conditions encountered in rearing sites where lumpfish are used to control parasites in salmon aquaculture.</p>