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A field study has been conducted in three semi-arid regions in Syria in order to define, under field conditions, the wheat yield response function to irrigation water salinity. The three regions were under similar climatic conditions but have different soil textures, varying between sandy loam to clay. The study involved 65 farms which had been using for more than 5 years, well water of different salinity values (0.44 to 14.1 dS/m) and which all applied the same cultural practices. Results indicate that the wheat (Triticum aestivium L.) yield response function to irrigation water salinity differs largely between the three regions. Wheat was more salt tolerant to irrigation water salinity in sandy loam soils than in the clay loam soils. The irrigation water threshold salinity value beyond which wheat yield started to decline, was 6.5 dS/m, 3.5 dS/m and 1.2 dS/m in the three regions, respectively. The lower threshold value was in the clay loam soil region. The three threshold values obtained in this study are different from the ''universal'' value of 4 dS/m which is proposed in current literature. The results demonstrate the necessity of establishing regional water quality criteria when planning water resource projects.