This paper focuses on the development of a quality index applied to conventional water treatment plants (WTPQI) with the objective of providing a more consistent way of evaluating performance and comparing different plants. The methodology, based on the Delphi method, relies on the same principles used at the beginning of the 1970s in the formulation of the Water Quality Index (WQI). Afterwards, the WTPQI was applied to ten different Brazilian conventional water treatment plants, with horizontal sedimentation basins and variable sizes (average flow rates range from 100–4,300 L s−1) and the results pointed out its usefulness as an instrument for plant evaluation. A significant correlation between plant performance and the WTPQI values was confirmed based on the filtered water turbidity, mainly for the rainy season when the raw water quality becomes harder to reach compliance with the drinking water standards. Therefore, these results open up the perspective of the use of the WTPQI as a reliable tool for the management of water supply systems.