A study of the processes of lipid peroxidation and the activity of the peroxidase enzyme, as well as photosynthetic pigments in susceptible tomato plants treated with salicylic acid (SA), during infection with the root-knot nematode Meloidogyneincognita. It was shown that, in the roots of SA-treated plants, the activity of lipid peroxidation is higher compared to the untreated plants, especially in the case of nematode invasion. A significant increase in the activity of lipid peroxidation in SA-treated invasive plants compared with untreated was noted during the transition of larvae to the sedentary stage and the beginning of the formation of feeding areas—giant cells (3–5 days after invasion). This, apparently, contributes to the inhibition of the development of the parasite and the reduction of plant infection and also indicates the involvement of oxidative processes in the mechanism of the induced resistance of plants to root-knot nematodes. In the SA-treated plants, the qualitative and quantitative composition of photosynthetic pigments, disturbed by invasion, was restored and corresponded to the control level.