We studied the biogeography of vascular plants on 10 islands in Laskeek Bay, Haida Gwaii (Queen Charlotte Islands), British Columbia. The islands varied in size from 4.5 to 395 ha and experienced a range of different browse pressures from introduced black-tailed deer (Odocoileus hemionus). We examined how island size interacted with browse pressure in determining the total species counts for individual islands. Numbers of plant species recorded increased with island area. The regression exponent for the log–log plot of species number on island area was 0.18, at the lower end of the range for such exponents. Many species absent from islands < 25 ha in area were characteristic of forest interiors, and consequently part of the increase in richness on larger islands probably was the result of increased forest interior area. Among the islands < 25 ha in area, the normal species–area and species–isolation relationships were reversed, with smaller, more isolated islands supporting more plant species than larger islands and, for a given area, more isolated islands supporting more species than less isolated ones. This reversal of the normal trend appears to be the result of deer browsing. Small, isolated islands were the only islands without deer and were richer, especially in wildflowers, than the larger, less isolated islands. On large islands, total species complement remained as predicted by area because the effect of deer was mitigated by the presence of deer-free refugia on cliffs and in isolated gullies. We conclude that deer are a major factor structuring the island plant communities and that continued protection of island habitats from introduced deer is essential to maintain the native flora of Haida Gwaii.