Dams are a critical component of infrastructure that provide vital services and protection to human communities and the economy, as well as safeguard from other adverse conditions. In Spain, there are approximately 1225 dams, of which 643 dams are more than 50 years old and 274 dams are located in areas of the high/very high seismic hazard class, and the exposure range is between $356–498 billion. This paper proposes a methodology to estimate the risk grade of collapse for dams that have been subjected to seismic processes. For this, a new set of importance factors (IFs) is identified and analysed. Three main concepts outline the methodology comprehensively: hazard, exposure, and vulnerability. Within this methodology, 5 classes, 17 factors and 47 subfactors were introduced for several dams. The results show that 46 dams could be classified as having a “very high/extreme risk”, with a total cost of repair of approximately $23 million. Currently, technical codes comprise very simplified risk concepts and obsolete IF definitions because they are only based on the construction type and its intended use. This study promotes the adoption of an innovative set of new IFs for the risk assessment of dams under earthquaking in a critical way.