Belowground competition is affected by the presence and identity of neighboring plants, as well as by environmental conditions. We examined the effects of the degree of relatedness (DOR) of neighboring Solanaceae relatives under salinity stress vs. control. Cherry tomato ( Solanum lycopersicum L.) (C) and bell pepper ( Capsicum annuum L.) (B) plants were grown individually or in pairs of high (H) DOR (CC and BB) and low (L) DOR (CB), under control and salinity conditions. In comparisons of plant responses to DOR and treatments, cherry tomato benefited from the presence of bell pepper, with increased CO ₂ assimilation (A), stomatal conductance (gs), plant height (H), shoot and root growth, xylem area and root respiration, thus acclimating better to salinity with L-DOR pairing. In contrast, salinity-stressed bell pepper showed impairment in A, gs, H, biomass, root anatomy, and proliferation of fine roots with significantly increased root respiration, especially with L-DOR pairing. Expression of genes in the tricarboxylic acid cycle (TCA) was also affected by the neighbor’s presence, influencing respiration rate. Acclimation to salinity is, therefore, species-specific and depends on the neighbor’s presence and DOR, suggesting that cultivating major crops with different DORs under extreme environmental constraints could increase stress tolerance for sustainable agriculture.