In the last decade, the publication of research studies in the field of groundwater ecosystem services and their classification has increased dramatically. Several academic and other institutions have developed ecosystem service classification systems for use in environmental policy research. However, the existing classification systems need to be optimized in the following areas: (a) the weak correlation between concepts and the definitions assigned, which creates double counting in economic valuation, (b) the absence of correlation of the concept of use value with the ecosystem service, (c) the non-categorization of final and intermediate services, and (d) the overlapping of wetland, soil, and groundwater regulating ecosystem services. Our research aims, through the results of a literature analysis and the synthesis of the results of an expert consultation process, to develop a unified categorization system for groundwater ecosystem services. In the context of the conceptualization of groundwater ecosystem services, this research implemented an expert judging elicitation process where subject discussions and targeted interviews were performed, combined with a literature review analysis. Through the completion of a specific questionnaire and expert interviews, a new groundwater ecosystem services classification system, namely GROUNDWES, was established.