To produce tomato fruits with 6% average Brix without decreasing yield, we investigated the effect of moderate salinity stress on Brix and yield in a single-truss, high-density tomato production system. Because tomato fruit development can be predicted from cumulative temperature, we also assessed cumulative temperature after anthesis as a potential indicator for determining the starting points of salinity stress treatments. When transverse diameters of the first fruit reached 4 cm (i.e., early increase treatment) or the first fruits were at the mature green stage (i.e., late increase treatment), nutrient solution electrical conductivity was slowly increased until the breaker stage from 1.8 dS·m⁻¹ to 6.0 dS·m⁻¹. Plants subjected to the late increase treatment produced tomato fruits with Brix values of 6% without reductions in marketable yield. We also increased nutrient solution electrical conductivity based on cumulative temperature after anthesis and found that early-treated plants produced tomatoes with higher Brix levels and yields than late-treated plants. In summary, moderate salinity stress to avoid excessive stress on plants increased sugar concentrations without decreasing fruit yield and resulted in tomato fruits with average Brix of about 6% when nutrient solution electrical conductivity was increased at a rate of 0.1 dS·m⁻¹·day⁻¹. Because seasonal differences in cumulative temperature influence the appropriate timing of salinity stress applications, further study is needed to optimize year-round growth under moderate salinity stress in single truss, high-density tomato production systems.