An unknown species of Roccellaceae Chevall. was collected in 2022 and 2023 on the bark of trunks in a rainforest in Basse-Terre island (Guadeloupe, Lesser Antilles). Phylogenetic analyses using mtSSU, nuLSU and RPB2 sequences reveal the placement of the species in the genus Mazosia A.Massal. (Roccellaceae). The lichen is unusual for the genus by being the first species of Mazosia developing a byssoid thallus. It is described as Mazosia byssoidea Lebreton & Ertz, sp. nov. Besides it byssoid thallus, the new species is characterized by 1-septate ascospores, stipitate pycnidia and a chemistry with roccellic acid. Together, all these features make the new species a distinctive and important addition to the Arthoniales Henssen ex D.Hawksw. & O.E.Erikss. So far, it is only known from two large trees in a dense humid forest at low elevation, a locality that needs to be included in a protected area. A key to the corticolous species of Mazosia is provided. Key words: Caribbean islands, Lesser Antilles, lichenized fungi, phylogeny, new species.