The Balu River, a critical water source in Dhaka, Bangladesh, is increasingly threatened by extensive pollution from industrial, agricultural, and domestic sources. This study assesses the water quality parameters of the river by water quality index approach, focusing on a 6 km stretch between Demra Ghat and Kamshair. Water samples were collected from five strategic points and analyzed for key parameters including pH, Turbidity, Chlorine, Phosphate, Iron, Color, CO₂, Dissolved Oxygen (DO), Biochemical Oxygen Demand (BOD), Total Dissolved Solids (TDS), Total Suspended Solids (TSS), Total Solids (TS), and Nitrate levels. The findings reveal that while some parameters meet acceptable standards, critical indicators such as DO, BOD, and nitrate exceed permissible limits in certain locations, reflecting severe pollution from untreated industrial discharge, agricultural runoff, and municipal waste. The calculated Water Quality Index (WQI) by National Sanitation Foundation (NSF) method ranges from 31 to 37, categorizing the river's water quality as bad. This degradation poses significant environmental and public health risks, especially as the river serves agricultural, domestic, and ecological functions. The study underscores the urgent need for stringent enforcement of pollution control measures, advanced wastewater treatment technologies, and public awareness initiatives to safeguard this vital water resource.