The brown bullhead (Ameiurus nebulosus) is a non-native, invasive fish species in Europe, and since it is sometimes present in high numbers in aquatic ecosystems, it may affect their functioning. The aim of the study was to determine the composition of the diet of A. nebulosus depending on its body size; the body size at which the brown bullhead becomes a predator and what share of its diet consists of fish. We determined the diet composition, frequency, and biomass of individual food types for 260 A. nebulosus individuals belonging to three classes in two shallow lakes. Analysis of the diet of A. nebulosus showed that it was highly varied, which manifested as a wide range of food types (15 types) as well as changes in its composition depending on the size of the fish. The nature of the habitat may influence the composition of the diet of A. nebulosus, which was most evident in the smallest individuals (total length <135 mm). Our observations showed that in Central European conditions, A. nebulosus with a length of more than 135 mm is able to feed on fish, and the share of fish in the food biomass increased with its total length, reaching even an 80% share of the biomass. In newly invaded areas, A. nebulosus can affect various groups of organisms by feeding on them. It can also compete with native fish species for food, but its main effect may be predation.