This paper addresses the role of pumped hydro storage (PHS) to decarbonization of the electricity sector using Spain's power system as a case study. Spain has an ambitious decarbonization target and a large installed base of pumped hydro. We conduct our analysis looking out to the projected load in 2030 and alternative portfolios of low-carbon generation capacity operated to meet that load. Our analysis will show how the existing capacity of pumped hydro improves the utilization of all low-carbon generation sources, including solar PV, wind, and also nuclear, while decreasing the dispatch of natural gas-red generation and therefore greenhouse gas emissions. We then evaluate the impact of additional investment in pumped hydro storage and how this impact varies as low-carbon sources become an even larger share of the system. Our results demonstrate that the expanding scale of low-carbon generation warrants additional investments in pumped hydro.