The watercress (Nasturtium officinale R. Br.) is often grown as a forced leaf vegetable rich in vitamins and minerals. Its specific taste and curative effects result from the presence of mustard glycosides (glucosinolates) that may also show nematocidic effects. The aim of this study was to evaluate the effect of watercress eluate and juice on tomato plants inoculated with the northern root-knot nematode (NRKN) Meloidogyne hapla. It was found out that after the treatment of plants with solutions derived from watercress the yield of fruit was highly significantly increased. The highest yield increase (by 96 % to 165 %) was observed in the variant with the highest concentration of watercress extracts (macerate + 200 ml of juice). Although at lower concentrations of watercress solution the increase in yields was also reduced (by 57–118 %), the difference was still statistically highly significant. The effect of this treatment on the content of vitamin C was similar and the increase was also highly significant and ranged from 24 to 67 %. When evaluating the occurrence of nematodes on roots of tomato plants, the presence of pathogens was equal to zero percent in all treated variants. The treatment with this phyto-solution corroborated not only a nematocidic effect of watercress but also its positive influence on the yield and level of vitamin C in dependence on increasing concentrations of the effective agent. It can be therefore concluded that the watercress solution showed a promoting effect on tomato plants.