Nowadays, continuous, healthy water supply and total water quality management have emerged as an important issue in engineering applications. Monitoring of all processes and their components at site requires studies on spatial base. Therefore, a Geographical Information System (GIS) based system was developed to construct the proposed Drinking Water Quality Monitoring Information System in this study for İstanbul. Establishing regular monitoring the quality of drinking water and effective reporting mechanisms to provide a relevant and timely information as well as to promote confidence in the water supply and its management are prior targets of the proposed system. The major purpose of this study was to monitor, evaluate and control the water quality in drinking water distribution system by utilizing GIS. Further in the study, a different approach is provided for water quality management. Thus, an efficient and dynamic management strategy is aimed to comply with drinking water standards. Drinking water quality monitoring is a wide-ranging assessment of the quality of water in the distribution system till it is supplied to the consumer. It includes the regular sampling and testing performed for assessing complaince with guideline values and, where applicable, compliance with regulatory requirements and/or agreed levels of service. The four main components of designed drinking water quality management system consist of Planning, Implementation, Efficient Monitoring and control and Continuous improvement steps. In this study a powerful and functional platform was projected for query, network and trend analyses, decision support, modelling, thematic re-symbolization, scenario production and management with GIS technology within the framework of “The Main Drinking Water Quality Management Plan” Application of this developed system was performed within İstanbul drinking water system. In order to monitor the water quality at influent and effluent of water treatment plants, water service reservoirs and selected critical points in the distribution system, a Geographical Information System was designed In this study, variations of water quality of water treatment plants, service reservoirs and distribution systems are evaluated individually. Moreover some inspections are made for each component of drinking water system according to charts based on measurement results. Besides, spatial variation of water quality is put forth by evaluating the timely data of water treatment plants, tanks and critical points in networks together. Some important decisions were derived from the developed system and the results of analyses are; . Control and efficiency of treatment processes, design of equipments, utilization of chemicals and monitoring procedure. . Development of new techniques related to repairment, rehabilitation, renewal of transmission lines, review of speed and quality of repairment / maintenance operations, management of water pressures in transmission lines, instrumentation and monitoring of the main water system, • Maintenance, repairment and cleanliness of water service reservoirs periodically, preventing human and animal, bird, vermin access to reservoir areas. • Water storage for emergency, providing required contact time for disinfection, reduction of dead points and pipe ends, contamination control at cross connections in the water distribution system, Decisions about drinking water quality should be based on available data and documents, such that the decisions are scientifically, and where necessary, legally defensible and able to withstand public scrutiny. Ready and easy accessible data is as important as making decision which has to be based on analysing the system data. Geographical Information Systems provide very powerful and efficient study platform to users in these terms. As a result, this study aims to put forth the benefits of a proposed Geographical Information System based decision support system which contributes to drinking water quality management.