<p>Salinity represents a growing issue for agriculture, hindering plant growth and crop productivity. Raffinose synthase-like seed imbibition protein (RS/SIP) is among the major genes involved in a wide variety of abiotic stresses; however, its function remains largely unknown. In this study, a grapevine <i>VvRS/SIP</i> was transferred to tomato plants via <i>Agrobacterium</i>-mediated transformation for its functional characterization under salt stress conditions. Transgenic tomato lines thus generated were screened at molecular and physiological levels to assess plant performance under salinity. Tomato seedlings were treated with different salt concentrations (0, 50, 100, and 150&#xa0;mM NaCl) for six weeks of vegetative growth in a greenhouse pot assay. Data confirmed that the increase in salt tolerance in <i>RS/SIP</i>-transgenic lines was found to be correlated with an increased <i>VvRS/SIP</i> gene expression level. Interestingly, <i>RS/SIP-</i>expressing lines triggered raffinose accumulation that reached 26-fold in the most tolerant transgenic line, against only eightfold in the wild-type (WT) plants. Physiological and biochemical analyses showed that <i>RS/SIP</i> plants exhibited higher raffinose, total soluble sugars, and proline accumulation, resulting in lower osmotic potential and improved water uptake. Additionally, transgenic lines showed improved K⁺ uptake, reduced Na⁺ accumulation, electrolyte leakage, and malondialdehyde (MDA), leading to better membrane stability and reduced cellular damage compared with WT under salinity. The results suggest a key role of <i>RS/SIP</i> in salt tolerance by boosting raffinose accumulation and osmotic adjustment to mitigate osmotic and oxidative stresses in tomato. The present investigation clearly highlights that <i>RS/SIP</i> is a promising candidate gene for producing salt-tolerant crops.</p>