The synapomorphic character of the genus Acarnus Gray, 1867 is the presence of the cladotylote spicule. Acarnus has 27 valid species worldwide, including five species reported from the Brazilian coast: A. innominatus Gray, 1867, A. nicoleae van Soest, Hooper & Hiemstra, 1991, A. souriei (Lévi, 1952), A. radovani (Boury-Esnault, 1973) and A. toxeata Boury-Esnault, 1973. Here we will describe A. microxeatus sp. nov., whose diagnosis is the presence of smooth microxeas and acanthoxeas, new characters for the genus. We also provide a complete characterization of the skeletal architecture and spicule morphology of A. toxeata, with scanning electron microscopy image (SEM) of the spicule set.