The crustose mucilaginous brown algae include genera from various orders (Ralfsiales, Nemodermatales and Ishigeales). To date only four species have been recorded in the Mexican tropical Pacific: Hapalospongidion gelatinosum, Mesospora pangoensis (Ralfsiales), Petroderma maculiforme and Diplura simulans (Ishigeales). This study provides a detailed description of a new genus and species, Johnawestia mexicana gen. & sp. nov. within the order Ralfsiales based on molecular and morphological analyses. Our phylogenetic analyses of rbcL, cox1, and concatenated rbcL-cox1 genes reveal that specimens collected in the Mexican tropical Pacific form a strongly supported clade within the family Hapalospongidiaceae that is distinct from other species, such as Hapalospongidion gelatinosum and Basispora africana. Johnawestia mexicana is characterized by crustose mucilaginous thalli consisting of mainly erect postigenous filaments and prostrate basal layers of primigenous filaments. Postigenous filaments originate by division of intercalary cells of the upper layer of the primigenous filaments, they are closely packed towards the base and loosen between one-third and one-half of the thickness of the thallus. Thick pedicels with a terminal unangium and lateral paraphyses both arise from the fourth to eighth cell of the erect postigenous filament. Pedicels with a terminal unangium can also arise directly from the basal layers. Plurangia are intercalary, below one or two terminal sterile cells. The new genus is represented by a species which is distinguished from the remaining species within the Hapalospongidiaceae by the thickness of its thallus, its postigenous cells packed only towards the base and the development of the unangia.