Seaside Sparrows (Ammodramus maritimus) nest at varying population densities, and breeding pairs may occupy either large, all-purpose activity spaces or small nesting territories, foraging in undefended areas separate from the nest site. We determined the prevalence of extra-pair paternity in a large, socially monogamous population of Seaside Sparrows nesting in small, overlapping territories. We used six microsatellite DNA markers and a likelihood-based approach to paternity assignment. Five of 47 chicks (11%) in three of 18 broods (17%) in this population were sired by extra-pair males. Although this is the first study of the genetic mating system in the genus Ammodramus, the rate of extra-pair paternity we observed is lower than in most other New World emberizines. As the first measurment of extra-pair paternity in Seaside Sparrows, this study provides a baseline for comparative studies of how extra-pair paternity is influenced by the wide variation in nesting density and territoriality found in Seaside Sparrows. These results, from a socially monogamous sparrow may also provide a context for studies of unusual mating systems in other salt-marsh nesting birds.