Plum Island, New York, has one of the highest concentrations of rare plants in the state and supports a high diversity of native plant species. The island's varied topography and diverse ecological communities have contributed to the diverse flora; for much of the past 100 years, approximately 90% of the island has remained undeveloped. The vascular flora consists of 414 species within 270 genera and 92 families. Largest families include Asteraceae (61 spp.), Poaceae (60 spp.), and Cyperaceae (26 spp.) and largest genera include Carex (9 spp.), Cyperus (8 spp.), and Juncus (7 spp.). Twenty three species are listed as endangered, threatened, or rare in New York. Of these 23 species, 17 are extant, having been observed in the field during this study and six are considered extirpated. Plum Island provides a significant refugium for biodiversity and should be preserved and protected.