Salinity is a type of abiotic stress that limits the growth and development of crops, including tomato (Solanum lycopersicum L.), one of the most consumed agricultural products worldwide. More and more agricultural soils contain high concentrations of salts due to inadequate agricultural practices and irrigation with low-quality water. At the same time, another environmental problem is the production of olive mill wastewater, which is characterized by its toxicity and high-volume production, affecting Mediterranean countries where olive oil is mainly produced. In this context, the aim of this work was to evaluate the efficacy of olive mill wastewater applied to tomato plants under salt stress conditions. For this purpose, an experiment was carried out in a greenhouse where tomato plants were kept under controlled conditions and two different doses of the olive mill wastewater were applied. Positive effects were obtained when the lowest dose was applied, increasing fresh weight by 25% and dry weight by 19%; oxidative stress effects such as MDA and H₂O₂ production and electrolyte leakage were reduced by 11, 12 and 12%, respectively; and proline content was also reduced by 14% compared to the saline control, so it had some efficacy as a biostimulant. However, the highest dose showed low efficacy as well as possible phytotoxic effects on tomato plants when this treatment was applied, producing an increase in transpiration of 51% compared to the normal control. This effect made the highest dose of olive mill wastewater unsuitable as a biostimulant under the conditions of this study.