Introduced species present one of the greatest threats to biodiversity of native species, and knowledge of introduced species ecology is imperative for the development of management plans to ensure conservation of native species populations. We sought to determine the distribution and nesting behavior of an introduced population of red-bellied squirrels (Sciurus aureogaster) on islands of the Florida Keys currently managed as part of Biscayne National Park, and document potential for the species to impact native flora and fauna. Squirrels were difficult to observe in the dense vegetation of the subtropical forest, so we relied on their leaf nests, which were highly visible in the canopy of trees, to determine current presence and distribution on the Park's islands. We found nests throughout the mixed-hardwood forests of Elliott Key and Sands Key, and also documented a single, old nest on Old Rhodes Key, the first ever documentation of the species that far south in the Upper Keys. Nests were located in tall trees with more canopy linkages than random focal trees, and nests were placed in the upper canopy on the north side of the nest tree more often than expected by chance. Squirrels selected West Indies mahogany (Swietenia mahagoni) to place nests more often than available in the forest. Squirrels used areas with greater tree density and canopy cover, but lower recent hurricane damage and fewer woody shrub stems, than areas available at random in the forest. Squirrels built nests only in mixed-hardwood forest. Overall, this introduced species exhibited nest site selection behavior similar to other tree squirrels, and appears capable of continued spread despite the initial site of introduction on an oceanic island. Knowledge obtained from this research is being used by managers and applied to an eradication program to remove this invasive species from Biscayne National Park.