Validation is important to the decision-making success and to the continued use of a Decision Support System (DSS). Without proper validation, DSSs may cause costly errors. However, little is known about available effective methods to validate such computer-based systems. In this paper a new method to validate DSS is presented. The approach combines several validation methods developed to validate Operations Research/Management Science (OR/MS) models and expert systems validation frameworks to define a practical guidance for the validation stage of the DSS development. A case study illustrates the effectiveness of the developed method.