Irrigation water quality is important to sustain agricultural productivity. The primary irrigation water sources in Al-Ahsa Oasis, KSA, are groundwater wells, mixed with treated wastewater and agricultural drainage. This study sought to evaluate irrigation water quality in Al-Ahsa Oasis with the aid of using irrigation water quality indices (IWQIs). One hundred and forty-eight different water samples were collected from various irrigation water resources throughout Al-Ahsa Oasis. The investigated physiochemical characteristics include pH, temperature, TDS, EC, turbidity, free chlorine, total hardness, cations (Na, K, Ca, Mg), anions (Cl, CO₃, HCO₃, SO₄), organic matter indices (DO, BOD, COD), and nutrients (NH₄, NO₃, PO₄). The IWQIs used in this study include salinity hazard, sodium adsorption ratio (SAR), Kelly’s ratio (KR), soluble sodium percentage (SSP), Permeability index (PI), residual sodium carbonate (RSC), and magnesium hazard (MH). The results indicated that treated wastewater mixed with groundwater is acceptable for irrigation. Spatial variations in irrigation water quality throughout Al-Ahsa are associated with water resources. For instance, groundwater mixed with agricultural drainage has the highest values of TDS, cations, and anions, whereas the lowest values were reported for treated wastewater, reflecting the good efficiency of wastewater treatment plants. The IWQI results revealed that 4.1% and 62.1% of the investigated irrigation water samples were considered good (class III) and satisfactory (class IV) for irrigation, respectively, whereas 33.8% of the collected water samples fall within the severe irrigation restrictions. Moreover, 79.7% of the investigated water samples were classified to have high to very high salinity hazards (C3, C4) and medium to high sodium hazards (S2, S3). Regular monitoring and assessment of treated water quality and wastewater treatment plant efficiency are important factors in achieving the sustainability of treated wastewater reuse in irrigation and consequently food security.