The diet of the marsh or water mongoose Atilax paludinosus has been well studied in coastal and inland riverine habitats, where crabs often constitute the main prey in terms of frequency of occurrence. We investigated the feeding ecology of a small number of marsh mongooses living next to a small, non-permanent reservoir (Andries Vosloo Kudu Nature Reserve, Eastern Cape), where freshwater crabs were not available. Using a combined metric of the percentage of occurrence and the percentage volume of food remains in 133 scats collected from 2006–2009, no primary prey could be detected. Amphibians, mammals, arthropods and fish all acted as secondary prey. Plants supplemented the diet, whereas birds only occurred as trace foods. There were seasonal variations in the diet, with peaks in amphibian (spring), arthropod (summer) and fish (autumn) consumption contributing to the change. Dietary diversity and niche breadth were relatively high throughout the year. This study strongly suggests that the marsh mongoose is in fact a generalist opportunist feeder. Although it consumes crabs and other aquatic prey in areas where they are particularly abundant, it can adapt to local food availability and include a significant proportion of terrestrial prey in its diet.