Between 2005 and 2011, we conducted a terrestrial invertebrate All Taxa Biodiversity Inventory (ATBI) in Boston Harbor Islands National Recreation Area, in order to document as many arthropod and gastropod species as possible in the park, and to understand how species were distributed across habitats and islands. Professional scientists, students, and citizen scientists collected ∼160,000 invertebrates on 19 islands and peninsulas in the park, using a variety of trapping and collecting methods. More than 76,000 of these specimens were curated, identified, and databased, resulting in a total of 1732 distinct species and morphospecies. Of these, 232 species (13.4%) were species not native to North America. The introduced species included several new US and North American records, including 2 potential pests: Hishimonus sellatus (Mulberry Leafhopper) and the click beetle Athous haemorrhoidalis. Among native species, we documented several new state records, which expanded known ranges considerably in a few cases. Statistical estimates of absolute species richness for several representative taxa indicated that less-diverse groups (e.g., millipedes) were sampled almost completely by our methods, but additional sampling is needed to thoroughly inventory more-diverse taxa (e.g., ground beetles). The invertebrate ATBI lays the groundwork for future monitoring of focal groups such as pollinators.