The response of tomato (Solanum lycopersicum L.) to abiotic stress has been widely investigated. Recent physiological studies focus on the use of osmoprotectants to ameliorate stress damage, but experiments at a field level are scarce. Two tomato cultivars were used for an experiment with saline water (6.57 dS m−1) and subsurface drip irrigation (SDI) in a silty clay soil. Rio Grande is a salinity-tolerant cultivar, while Heinz-2274 is the salt-sensitive cultivar. Exogenous application of proline was done by foliar spray at two concentrations (10 and 20 mg L−1) during the flowering stage. Control plants were treated with saline water without proline. Proline at the lower concentration (10 mg L−1) increased dry mass of different plant organs (leaves, stems, and roots) and it improved various chlorophyll a fluorescence parameters compared with controls. Regarding mineral nutrition, K+ and P were higher in different organs, while low accumulation of Na+ occurred. However, Mg2+ was very high in all tissues of Rio Grande at the higher concentration of proline applied. Thus, the foliar spray of proline at 10 mg L−1 increased the tolerance of both cultivars. The growth of aboveground biomass of Heinz-2274 was enhanced by 63.5%, while Rio Grande improved only by 38.9%.