Zanthoxylum thomasianum, also known as the St. Thomas prickly-ash, is a species of plant in the Rutaceae family. It is found in Puerto Rico, the British Virgin Islands, and the United States Virgin Islands. Its natural habitats are tropical and subtropical dry broadleaf forests and shrublands. It is threatened by habitat loss, and is the only on St. John listed as "endangered".Some taxonomers consider it a subspecies of Zanthoxylum punctatum.