The United States National Park Service protects and preserves iconic landscapes and their associated and unique natural and cultural resources across the country. Although many of these national parks, particularly in the eastern US, were established for their historical importance, these small, protected areas can house notable biological diversity. The Chesapeake and Ohio Canal National Historical Park, an historic park within and proximate to the nation's capital, has long been the subject of botanical surveys, in part because of the diverse plant communities it contains. Herein, we synthesized historic botanical data on rare plant species within the park and summarized 14 years of recent data-collection efforts. Of the 191 rare, threatened, and endangered plant species historically documented, we identified 105 species that contain current populations within the park, but we were unable to relocate the remaining 86 species. Based on the State of Maryland species conservation ranks, critically imperiled species were no more likely to be documented during our study than were historically recorded species, potentially indicating that more species have become locally extirpated than previously known. Despite the intensity of local land-use and anthropogenic stressors, urban–suburban protected areas such as the subject national park are critical for the integrity of rare-plant populations. We provide descriptions of the unique plant species, communities, and conservation threats.