In Tanzania’s water sector, where only 54% of urban populations enjoy reliable access (World Bank, 2023), service reliability remains a critical yet understudied driver of customer satisfaction, particularly in semi-rural utilities. This study examines the impact of service reliability measured through water supply consistency, billing accuracy, and complaint resolution on customer satisfaction at Njombe Urban Water Supply and Sanitation Authority (NJUWASA). Using the SERVQUAL model, data were collected from 385 stratified-sampled customers via structured questionnaires. SPSS analysis revealed a strong positive correlation between reliability and satisfaction (r = 0.78, p < 0.01), with reliability explaining 61% of satisfaction variance (R² = 0.61). Billing reliability was a strength (mean = 3.4/5), but frequent interruptions (68% of customers monthly) and delayed complaint resolution (mean = 2.5/5) undermined satisfaction. NJUWASA should prioritize infrastructure upgrades, real-time communication (e.g., SMS alerts), and community engagement to align with Sustainable Development Goal (SDG) 6’s targets for equitable water access. This study offers a replicable framework for utilities in resource-constrained settings, emphasizing proactive maintenance and inclusive governance to bridge service gaps.