Background <p>Metal pollution in agricultural soils is imparting devastating influence on the growth and yield of crop plants. Therefore, role of gibberellic acid (GA₃; 100 mg L^− 1) priming and the foliar spray of spermine (Spm; 200 µM) in mitigating the effects of mercury (Hg; 20 µM) stress on growth, photosynthesis, nitrogen metabolism and nutrient uptake were studied in tomato.</p> Results <p>Mercury (Hg) significantly reduced plant height, fresh and dry weight, chlorophylls, carotenoids and photosynthetic parameters significantly however, GA₃ and Spm treatments alleviated the decline considerably. Oxidative damage parameters were reduced notably by GA₃ and Spm with their combined treatment imparting higher decline. The decline in nitrogen metabolism enzymes was assuaged by GA₃ and Spm. Content of compatible osmolytes further increased due to GA₃ and Spm treatments. Treatments of GA₃ and Spm increased the activities of γ-glutamyl kinase and proline oxidase causing increased proline buildup. Moreover, the activities of antioxidant and glyoxylase cycle enzymes, and the content of ascorbic acid, reduced glutathione and cysteine increased significantly due to the GA₃ priming and Spm supplementation. Total phenols and the total antioxidant activity interms of DPPH and ABTS radical scavenging increased due to GA₃ and Spm treatments. Exogenous GA₃ and Spm reduced the accumulation of Hg and alleviated the decline in beneficial nutrients.</p> Conclusion <p>The oxidative effects of Hg stress were alleviated by GA and Spm treatments, more apparently due to their combined treatment. Results reflect the interactive effect of GA and Spm on the important tolerance mechanisms.</p>