<p>Copper (Cu) toxicity is a major abiotic constraint that reduces plant growth and productivity. This study examined whether exogenous allantoin can alleviate Cu-induced oxidative damage in rapeseed (<i>Brassica napus</i> L.) seedlings. Seedlings were treated with 50 µM Cu alone or combined with foliar allantoin applications (10, 50, 75, 100 µM). Growth traits, chlorophyll content, oxidative stress markers, and antioxidant enzyme activities, superoxide dismutase, catalase, ascorbate peroxidase, and glutathione reductase, were measured. Non-enzymatic antioxidants, ascorbate and glutathione, proline, phytochelatins, and Cu/Fe concentrations were also quantified. Cu stress markedly reduced root and shoot biomass (62% and 55%) and decreased chlorophyll content, while increasing hydrogen peroxide and malondialdehyde by nearly threefold. Allantoin treatments partially reversed these effects. At 100 µM, allantoin increased root and shoot biomass by 32% and 47%, lowered hydrogen peroxide and malondialdehyde levels by 57% and 54%, and enhanced total chlorophyll by 58% compared with Cu alone. Allantoin also elevated enzyme activities, improved ascorbate and glutathione redox status, increased proline, stimulated phytochelatin synthesis (52% higher than Cu alone), and reduced Cu translocation to shoots. Fe content in shoots, which had declined by 54% under Cu stress, was partially restored. Exogenous allantoin reduced Cu-induced oxidative injury and improved tolerance through enhanced antioxidant activity, redox regulation, osmoprotection, and metal detoxification. These results suggest a potential utility for allantoin in improving stress resilience in rapeseed; however, further molecular and field studies are required to validate broader applications.</p> Graphical Abstract <p></p>