The increase in inappropriate agricultural practices and irrigation with low-quality water has led to a rise in the amount of soil with high salt concentrations. Salinity is a type of abiotic stress that significantly affects the growth and development of crops, including tomato (Solanum lycopersicum L.), which is one of the most consumed agricultural products worldwide. One way to mitigate the problems caused by salinity can be the use of biostimulants. Bioactive compounds and secondary metabolites, such us phenolic acids, glycosides, and vitamins mean that the horsetail plant (Equisetum arvense L.) can have a biostimulant effect on crops. Also, it accumulates large amounts of Si, which is beneficial for plant growth, the photosynthetic rate, and can help to alleviate the effects of biotic and abiotic stresses. In this context, the aim of this study was to evaluate the effectiveness of Equisetum arvense applied to tomato plants under saline stress conditions. For this purpose, an experiment was carried out in a greenhouse where tomato plants were grown in hydroponics under controlled conditions. First, macronutrients and micronutrients, silicon and phenolic compounds, were extracted through magnetic stirring, and then, the E. arvense extract (EA-E) obtained was applied via foliar feeding to tomato plants to test its efficacy. Positive results were observed after spraying the extract (EA-E). The treatment with the extract (EA-E) was established according to the concentration of phenolic compounds and silicon. It induced an increase in biomass production and chlorophyll synthesis; fresh weight increased by 24% compared to the saline control (SC); and the chlorophyll concentration reached the values of the normal control (NC). In terms of oxidative stress, EA-E was also effective; MDA and H₂O₂ concentrations were reduced by 57% and 29% and the proline concentration decreased by 23% compared with the saline control (SC). These results show the effectiveness of E. arvense extracts (EA-Es) in reducing the effects of salinity on tomato plants under the conditions of this study. For tomato, Equisetum arvense, under salt stress, was used as the biostimulant in this study.