This paper presents a comprehensive method for assessing the technical condition of water treatment plants as part of a set of procedures for assessing the entire water supply system. The proposed method of water treatment plant assessment is based on a system of technical indicators and factors. Based on these, the assessment of the technical condition is carried out as a two-stage process, where the evaluator first determines discrete values of the factors. The point score given to these factors is then aggregated and used to calculate the values of individual indicators, indicating the overall condition of the water treatment plant. The proposed set of assessment indicators therefore includes two types of indicators: structural–technical and technological–operational. The paper includes a case study demonstrating the use of the proposed technology to assess the technical condition of an actual water treatment plant. This methodology makes it possible to highlight critical parts of the system and, as the case may be, prioritise the elements in terms of their technical condition, which can then be used in the repair and reconstruction planning process.