The floristic diversity and vegetation communities of Leonard Marsh and Harpers Meadow, two peatland complexes of recent origin within the Lake Umbagog National Wildlife Refuge in Coos County, New Hampshire, were investigated. The combined vascular floras of Leonard Marsh and Harpers Meadow consisted of 178 species, 111 of which occurred in both peatlands. The flora included two state-listed species, Eriophorum angustifolium subsp. angustifolium (endangered) and Listera cordata (threatened). Abundance and frequency data from 617 quadrats along 35 transects were analyzed by two-way indicator species analysis (TWINSPAN) and resulted in the recognition of nine vegetation cover types (CT): Thuja occidentalis–Alnus incana CT, the Picea mariana–Nemopanthus mucronatus CT, the Nemopanthus mucronatus–Alnus incana CT, the Rhododendron canadense–Nemopanthus mucronatus CT, the Rhododendron canadense–Chamaedaphne calyculata CT, the Chamaedaphne calyculata–Carex oligosperma CT, the Scheuchzeria palustris–Carex oligosperma CT, the Carex lasiocarpa–Chamaedaphne calyculata CT, and the Dulichium arundinaceum–Potentilla palustris CT. A tenth cover type consisting of aquatic vegetation, the Brasenia schreberi–Potamogeton spp. CT, was recognized based on qualitative field observations. Despite the recent formation of these peatland complexes following the construction of the Errol Dam in 1853, the vegetation composition of Leonard Marsh and Harpers Meadow shares broad similarities with peatlands in New England, the northcentral United States, and adjacent Canada. Leonard Marsh and Harpers Meadow are best classified as oligotrophic to weakly minerotrophic level peatland complexes occupying broad and shallow outwash plains. Historical records, botanical evidence, and patterns in basin topography suggest that terrestrialization and paludification have been critical processes in the post-settlement development of these two large peatland complexes.