In scope of crop salinity tolerance, an experiment was carried out in a field using saline water (6.57 dS m−1) and subsurface drip irrigation (SDI) on two tomato cultivars (Solanum lycopersicum, cv. Rio Grande and Heinz-2274) in a salty clay soil. Exogenous application of proline was done by foliar spray at two concentrations: 10 and 20 mg L−1, with a control (saline water without proline), during the flowering stage. Significant higher increases in proline and total soluble protein contents, glutamine synthetase (GS, EC6.3.1.2) activities and decreases in proline oxidase (l-proline: O2 Oxidoreductase, EC1.4.3.1) activities were detected in both tomato cultivars when irrigated with saline water (6.57 dS m−1) and exogenously applied by the lower concentration of proline. Taking in consideration the obtained results, it was concluded that the foliar spray of low concentration of proline can increase the tolerance of both cultivars of tomato to salinity under field conditions.