Recent research has considerably advanced our ability to model extreme surges. Yet, simulating unprecedented events, i.e. events which are more extreme than observed in historical datasets, remains challenging. To some extent, anticipating such events is possible by accounting for the full range of climate variability. Using a 525-year synthetic dataset from a seasonal reforecast archive, this study uncovers potential unprecedented storm surge events for European and Mediterranean coastlines, focussing on their magnitude, spatial extent and seasonality. We identify Germany, the Netherlands and western UK as hotspot regions that could experience unprecedented storm surge levels that are more than half a meter higher than historical events. Spatially unprecedented extreme peak surges, affecting more provinces within a 3-day time window than previously recorded, may impact up to 49% of the provinces, with simultaneous effects in the Mediterranean and the Atlantic regions. Additionally, countries including the Netherlands, Germany and the UK may experience at least one temporally unprecedented surge during the summer months. Understanding these different dimensions of unprecedented events represents a significant advance in our knowledge of coastal flood risk in Europe and supports improved coastal flood risk management decisions, including enhanced flood defence standards, disaster risk management and planning of coastal operations.