The Saq aquifer is a Cambro-Ordovician Sandstone Formation that extends over 1200 km in Saudi Arabia and northwards in Jordan. It is an enormous groundwater reservoir where water is found under both unconfined and confined conditions. The main concern of this study was the quality of the Saq aquifer groundwater. This includes a description of the chemical composition of the groundwater, the spatial variation of its salinity and the processes that control this variability. A total of 384 wells drilled in the Saq aquifer were sampled and water analyses performed for the major ionic composition. The data were analysed using software computer programs. Saturation indices were calculated, ionic relationships were studied, hydrochemical facies were determined and water types were identified. The principal processes that control the groundwater quality variations are also discussed. The results of this study provide information needed for groundwater quality management in the Saq aquifer.