Abstract <p>The effect of seed priming with gold nanoparticles (15 nm, 20 µg/mL) on photosynthetic apparatus, pro-/antioxidant balance and cold tolerance of tomato (<i>Solanum lycopersicum</i> L., var. Kulon) was examined for the first time. Nanopriming enhanced the levels of chlorophylls and carotenoids, as well as increased the relative expression of the genes (<i>rbcL</i> and <i>rbcS</i>) encoding the large and small subunits of ribulose-1,5-bisphosphate carboxylase/oxygenase. In addition, the activity of antioxidant enzymes (superoxide dismutase, guaiacol peroxidase, catalase) and the content of low molecular weight antioxidants (carotenoids, anthocyanins) were increased in tomato leaves. In cold conditions (9°C, 5 d) nanopriming decreased malondialdehyde and H₂O₂ content in leaves, while increased the activity of guaiacol peroxidase and superoxide dismutase. As a result, the cold tolerance of tomato evaluated by electrolyte leakage from leaf tissues was increased markedly compared to control (untreated with nanoparticles) plants. This study suggested the potential of gold nanoparticles as a promising strategy to mitigate cold stress in tomato plants by affecting photosynthesis and antioxidant system.</p>