A new saprotrophic species was discovered during our fungal inventory at the Boston Harbor Islands National Recreation Area (Massachusetts), which consists of 34 islands and peninsulas. Simocybe ramosa sp. nov. (Agaricales, Crepidotaceae) is described based on morphology and molecular phylogenetic data. The holotype collection was found in a Quercus (oak)–Carya (hickory) forest under bark of a dead oak tree on World's End peninsula, the largest land mass of the archipelago. Phylogenetic reconstruction of a dataset of the internal transcribed spacer region (ITS) resolved S. ramosa and S. rhabarbarina as sister species. Simocybe rhabarbarina is here redescribed based on the holotype and newly reported material from the Netherlands, and its presence on the island of Jersey, off the coast of northern France, is confirmed based on an ITS sequence. Finally, we compare morphological features of S. ramosa with S. rhabarbarina and the 20 species in the genus that have thus far been recorded in Canada, the US, and Mexico.