Coastal zones are particularly vulnerable to flooding. Several climatic and state variables may drive the occurrence of such events, e.g., storm surges, sea level rise, heavy rainfall, and high river and groundwater levels. The co-occurrence of such events, i.e. compound or cascading effects, has been shown to escalate flooding impacts and extent, but the contribution of groundwater is routinely overlooked. Here, we apply an integrated hydrological/hydrodynamic/groundwater model to investigate underlying causes and compound effects in a Danish Wadden sea catchment. Two models were developed: a long-term model and an overbank-spilling model. The long-term model was calibrated and used to simulate 30-year periods. Extreme value analyses were carried out for sea levels, precipitation, simulated river water stages, and groundwater levels. The co-occurrence of extremes was used to identify compound effects on high river-stage incidents (as a flood proxy). The overbank-spilling model was then used for simulating flooding for a subset of the largest river stage events identified from the long-term model. The analysis showed that the river-stage events were closely correlated to the sea level extremes, but that the largest river-stage events were almost exclusively compounded by precipitation or groundwater, or both. High groundwater tables seem to correlate to the flooding events with the largest spatial extent, as well as prolonged extreme events where either precipitation or sea level were elevated during long periods. Thus, this study shows that there is a general need to acknowledge the potential effect of groundwater levels on the resulting flooding on terrain in coastal zones.