This study develops an alternative methodology for the risk analysis of information systems security (ISS), an evidential reasoning approach under the Dempster-Shafer theory of belief functions. The approach has the following important dimensions. First, the evidential reasoning approach provides a rigorous, structured manner to incorporate relevant ISS risk factors, related countermeasures, and their interrelationships when estimating ISS risk. Second, the methodology employs the belief function definition of risk—that is, ISS risk is the plausibility of ISS failures. The proposed approach has other appealing features, such as facilitating cost-benefit analyses to help promote efficient ISS risk management. The paper elaborates the theoretical concepts and provides operational guidance for implementing the method. The method is illustrated using a hypothetical example from the perspective of management and a real-world example from the perspective of external assurance providers. Sensitivity analyses are performed to evaluate the impact of important parameters on the model's results.