Greater celandine (Chelidonium majus L., GC) exhibits a range of biological activities that support its traditional use in folk medicine and highlight its potential therapeutic applications in modern healthcare. This study was designed to investigate the effects of GC root and stem extracts on lipid peroxidation, oxidative protein modification, total antioxidant capacity (TAC) and antioxidant enzyme activities (superoxide dismutase, SOD, catalase, glutathione peroxidase, GPx) in Atlantic sturgeon muscle tissue after in vitro treatment. The plant material (stems and roots) was collected from natural habitats on the territory of the South Park in Słupsk, Pomeranian Province (northern part of Poland). The supernatant obtained from Atlantic sturgeon (Acipenser oxyrinchus oxyrinchus Mitchill) muscle tissue was used for incubation with GC root and stem extracts in vitro (at a final concentration of 5 mg•mL -1 ) at room temperature. The untreated control samples (muscle tissue) were incubated with 100 mM Tris-HCl buffer (pH 7.2) in the same way as the GC-treated samples. The incubation time was 2 hours. Biomarkers of lipid peroxidation and protein oxidation, TAC, and antioxidant enzyme activities were analysed in the incubated homogenates (control untreated samples and samples treated with GC root and stem extracts). We observed an increase in the levels of aldehyde and ketone derivatives of oxidatively modified proteins with a simultaneous increase in TAC levels after in vitro incubation of sturgeon muscle tissue with GC stem and root extracts. The intensity of lipid peroxidation was not altered. SOD and catalase activities were statistically significantly decreased, while GPx activity was not altered, suggesting the maintenance of antioxidant activity of sturgeon muscle tissue by this antioxidant enzyme. The results of this study highlight the pro-oxidative potential of GC extracts at the doses studied as potent natural oxidants to enhance antioxidant defences, but these processes are accompanied by the induction of oxidative protein modification in sturgeon muscle tissue. Future research should focus on elucidating the specific bioactive compounds responsible for the observed effects, optimising extraction methods to enhance the bioavailability of antioxidants, and evaluating the long-term effects of dietary supplementation with GC extracts on fish performance and product quality.