In today’s world of dynamic changes in the business environment and intense competition, the key to sustainable competitive advantage lies in delivering high quality services, which will in turn result in satisfied customers and consequently lead to customer retention and business sustainability. Service quality has become an area of interest in the service industry and more importantly, for the financial industry which has undergone evolution in light of technology and the financial market dynamics. Service quality is a significant determinant of customer satisfaction, and an important managerial issue in competitive market. The ability of an organization like a bank, to determine and meet customers’ requirements is key to influencing their continued relationship with the organization. This in turn leads to profitability and sustainable growth for the banks. The purpose of this study was to assess the effect of service quality on customer satisfaction at the commercial banks in Nairobi County, Kenya. To achieve this purpose, data was collected randomly from 401 respondents, selected from the general population of customers with bank accounts in the Tier I commercial banks in Nairobi. The study employed and used a structured questionnaire to collect primary data from the bank customers and employed descriptive and correlational research design. Bank Service Quality (BSQ) model was adopt the as a measure of service quality. The study findings indicated that service quality has an effect on customer satisfaction. The Spearman correlation analysis revealed a strong association between service quality and customer satisfaction. The study findings also indicated that the commercial bank customers were satisfied with banks services. The most valued service quality dimension was reliability. The population of the study was limited to bank customers in Nairobi county, future research can therefore be conducted in the other counties in Kenya. The study can also be replicated in other industries.