A new hybrid microsoroid fern, Microsorum x tohieaense (Microsorum commutatum x Microsorum membranifolium) from Moorea, French Polynesia is described based on morphology and molecular phylogenetic analysis. Microsorum x tohieaense can be distinguished from other French Polynesian Microsorum by the combination of sori that are distributed more or less in a single line between the costae and margins, apical pinna wider than lateral pinnae, and round rhizome scales with entire margins. Genetic evidence is also presented for the first time supporting the hybrid origin of Microsorum x maximum (Microsorum grossum x Microsorum punctatum), and possibly indicating a hybrid origin for the Hawaiian endemic Microsorum spectrum. The implications of hybridization for the taxonomy of microsoroid ferns are discussed, and a key to the microsoroid ferns of the Society Islands is provided.