The vascular flora of Timber Island, the largest undeveloped island in Lake Winnipesaukee, New Hampshire, was surveyed, and a vegetation analysis conducted. The island was found to have 187 vascular plant species, none of which are listed as rare or endangered for the state. The combination of tree species found throughout Timber Island indicates a forest type characteristic of the Central New Hampshire region. In the vegetation analysis, abundance and frequency data were recorded from 106 study plots and analyzed using a two-way indicator species analysis program (TWINSPAN). Six cover types (CT) were defined: Pinus strobus–Gaylussacia baccata CT, Fagus grandifolia–Ostrya virginiana CT, Pinus resinosa–Gaylussacia baccata–Vaccinium angustifolium CT, Tsuga canadensis CT, Acer rubrum–Dulichium arundinaceum CT, and Ruderal CT. Two additional cover types were recognized through qualitative field observation alone: Shoreline CT, and Vernal Woodland Pool CT. While not included in the floristic analysis, twenty-four bryophyte species were documented on the island, 15 of which are new records for Belknap County. A modified Line Intercept Analysis, employed to quantify ground cover, showed that herbaceous plant and bryophyte cover was sparse (7% each), with duff and leaf litter covering 74%, exposed rock outcrop 8%, vernal pools 3%, and 1% disturbed or bare soil. Soil analysis revealed a weak correlation with cover type. Two indices of similarity, Sørensen's Index and the Simple Matching Index, were utilized to compare the vascular plant flora of Timber Island to three other island floras in Lake Winnipesaukee: Rattlesnake Island, Bear Island, and Three Mile Island. Sørensen's Index showed a 50.0% similarity with Bear Island, 51.1% with Rattlesnake Island, and 52.7% with Three Mile Island. The Simple Matching Index showed higher levels of similarity: Bear Island, 51.66%; Rattlesnake Island 59.51%; and Three Mile Island 65.59%.