Based on collections assembled by Dutch expeditions to the Northwest African region, including the offshore islands in the neighbouring Atlantic, a taxonomic monograph of sponges of the genus Mycale is presented. Additional material from the region borrowed from the Zoologisk Museum of the University of Copenhagen and incidental samples made by individual collectors were also included. The combined collections contained sixteen species, nine of which are new to science: Mycale (Aegogropila) syringosimilis spec. nov., Mycale (Aegogropila) tenerifensis spec. nov., Mycale (Arenochalina) africamucosa spec. nov., Mycale (Carmia) atropha spec. nov., Mycale (Carmia) guineensis spec. nov., Mycale (Naviculina) cruzi spec. nov., Mycale (Paresperella) janvermeuleni spec. nov., Mycale (Rhaphidotheca) verdensis spec. nov. and Mycale (Zygomycale) sierraleonensis spec. nov. We briefly reviewed six Mycale species known from the region, but not represented in our material, making the faunal diversity twenty-two species. We present a key to all species of Northwest Africa and the neighbouring offshore Atlantic islands. The regional diversity of Mycale species is discussed in comparison to those of other regions.