A population of wild boars (Sus scrofa) became established during the 1990s in the Natural Park of Aiguamolls de l'Empordà, a wetland on the western coast of the Mediterranean Sea (Catalonia, Spain). Between 2001 and 2004, a culling program was conducted to reduce the boar population. We collected and analyzed the contents of 142 stomachs to characterize boar diets, estimate impacts on ground-nesting birds, especially threatened species, and determine the relationships between boars and agricultural areas surrounding the park. The boar population consumed primarily plant material (94% by volume), particularly underground roots and rhizomes (33%). Agricultural (37%) and non-agricultural plants (49%) comprised very similar proportions of the diet. The primary foods were alkali bulrush (Scirpus maritimus) (24% by volume, and in 47% of the stomachs) and corn (Zea mais) (19% by volume and in 29% of the stomachs). Animal matter represented only 5.6% of the diet by volume, but occurred in 84% of the stomachs. The most important animal foods were, by volume, birds (2.3%) and crayfish (Procambarus clarkii) (1.7%) and, by frequency, snails (44%) and terrestrial arthropods (47%). Wild boar diets shifted seasonally between agricultural plants in summer and non-agricultural plants and acorns (Quercus sp.) in winter. Consumption of animal matter varied seasonally, crayfish were consumed primarily from May to October, terrestrial arthropods in May and June, and birds from March to April and September to October. Birds, especially ducks, were consumed most frequently while moulting, when vulnerable to predation. Given the high frequency of birds in the diet and the extensive rooting for underground parts of plants, the wild boar population might pose a threat to the coastal wetland ecosystem of the Natural Park of Aiguamolls de l'Empordà if allowed to become overabundant.