We studied the diet of Black Skimmers (Rhynchops niger) through analysis of pellets and faeces collected at Patos Lagoon estuary, southern Brazil, where non-breeding birds gather from December to August. We identified 23 prey types from twelve families of fishes, two of crustaceans, and one of squid (N = 1797 prey items). Silversides (Odonthestes argentinensis and Atherinella brasiliensis), anchovies (mainly Lycengraulis grossidens and Engraulis anchoita), and mullets (Mugil spp.) were the main components of the diet, representing 83% of number and 81% of mass of prey. Prey size ranged 9.7-212.1 mm total length and 0.01-78.1 g, with mean at 51.6 mm and 2.81 g. Considering frequency of occurrence and the relative contribution in number and mass, the Argentinean Silverside (O. argentinensis) was the most important prey in three seasons of the year. Anchovies represented 41% of ingested mass in summer. Small-sized mullets represented 30% of prey number; but showed low mass contribution (maximum of 13% in winter). Sciaenids were only important in autumn (18% of ingested mass). Otoliths were found in 53% of pellets and 55% of faeces samples, suggesting that the relative contribution of smaller-sized prey to the diet was underestimated. Composition of diet and size distribution of prey suggest that Black Skimmers fed mainly in the lower estuary, but also on the adjacent ocean beach.