Abstract There is increasing evidence showing that low selenium (Se) concentrations may increase tolerance of crop plants to several environmental stresses. The aim of this study was to compare the influence of two chemical forms of Se (selenite or selenate) at different concentrations (2 or 6 μM) on the resistance of butterhead lettuce (Lactuca sativa L. var. capitata) cv. Justyna to NaCl-induced stress (40 mM NaCl). Plant growth was negatively affected by salinity, but the level of photosynthetic pigments was not reduced. Se application at a concentration of 2 μM significantly improved the growth of salt-stressed plants, but selenite was much more effective than selenate in enhancing salt-tolerance of lettuce. The growth-promoting effect of Se was also noted at 6 μM of selenite, but did not appear at 6 μM of selenate. The beneficial effect of Se in salt-stressed lettuce could be due to antioxidative activity of Se, root system growth stimulation, and/or increase in photosynthetic pigment concentration after Se supplementation; however, it was not related to either increase in proline accumulation or reduction in foliar Na+ or Cl− concentration. These results imply that Se application, especially in the form of selenite, can enhance antioxidant defense of lettuce under salt stress, and Se supplementation may be recommended for areas of lettuce cultivation with excessive salt accumulation.