We aimed to assess the effects of spirulina supplementation during gradual weight loss on serum concentrations of follistatin (FST), myostatin (MST), insulin-like growth factor 1 (IGF-1), aspartate aminotransferase (AST), alanine aminotransferase (ALT) and body composition in competitive wrestlers. Forty competitive wrestlers (age: 22 (sem 2) years) were randomly assigned to one of two groups: gradual weight loss + spirulina (SP; n 20) or gradual weight loss + placebo (PL; n 20). Subjects in both groups lost weight according to a designed diet over 12 d and were required to reduce baseline body mass (BM) by 4%. Subjects in the SP group received two tablets of spirulina, while subjects in the PL received two tablets of placebo before each meal. Concentrations of mentioned serum markers and body composition were measured before and after the interventions. BM (SP = −3·1 kg and PL = −2·9 kg), body fat percentage (BFP) (SP = −2·1 % and PL = −0·6 %), fat mass (FM) (SP = −2·2 kg and PL = −0·9 kg) and skeletal muscle mass (SP = −1·4 kg and PL = −1·5 kg) significantly decreased in both groups (P < 0·05). The changes in BFP and FM were significantly greater in SP compared with the PL group (P < 0·001). Additionally, MST (SP = −0·1 ng/ml), AST (SP = −2·1 u/l) and ALT (SP = −2·7 u/l) concentrations significantly diminished in SP group (P = 0·005), while FST (PL = −0·1 ng/ml) and IGF-1 (PL = −2·6 ng/ml) concentrations significantly decreased in PL group (P < 0·05). Spirulina supplementation during gradual weight loss is beneficial in reducing BFP, FM, MST and liver enzymes while maintaining IGF-1 and FST concentrations in competitive wrestlers.