This study evaluates the seasonal variation in physicochemical and biological characteristics of the Gandak River and assesses its suitability for drinking purposes with reference to standards prescribed by the Bureau of Indian Standards under IS 10500:2012. Surface water samples were collected from selected stations during summer, monsoon, and winter seasons and analyzed for pH, turbidity, total dissolved solids (TDS), conductivity, total hardness, alkalinity, major ions (Ca²⁺, Mg²⁺, Na⁺, K⁺, chlorides, sulphates, phosphates), dissolved oxygen (DO), biological oxygen demand (BOD₅), and microbial contamination (MPN). The results demonstrated significant seasonal variability. Most chemical parameters, including pH, TDS, chlorides, and sulphates, remained within acceptable and permissible limits, indicating moderate mineralization and limited industrial influence. However, turbidity exceeded recommended standards across seasons, particularly during monsoon, reflecting enhanced sediment load and catchment runoff. Elevated hardness and alkalinity during summer suggest geogenic contributions and possible domestic wastewater inputs. Although phosphate concentrations were relatively low (0.02–0.07 mg/L), their presence indicates nutrient enrichment that may support eutrophic conditions. DO levels showed seasonal fluctuation, with comparatively lower values during summer and monsoon corresponding to higher BOD₅ levels, suggesting increased organic pollution. Elevated MPN counts at several stations indicate substantial microbial contamination and potential public health risks. Overall, while most chemical parameters comply with IS 10500:2012 standards, high turbidity and microbial load render the river water unsuitable for direct consumption without treatment. The aim of this study was to assess seasonal water quality variations of the Gandak River, evaluate compliance with national standards, and identify anthropogenic factors influencing its ecological health.