S Mus is the Cepheid with the hottest known companion. The large ultraviolet flux means that it is the only Cepheid companion for which the velocity amplitude could be measured with the echelle mode of the HST GHRS. Unfortunately, the high temperature is difficult to constrain at wavelengths longer than 1200 Å because of the degeneracy between temperature and reddening. We have obtained a FUSE spectrum in order to improve the determination of the temperature of the companion. Two regions that are temperature sensitive near 16,000 K but relatively unaffected by H2 absorption (940 Å and the Lyβ wings) have been identified. By comparing FUSE spectra of S Mus B with spectra of standard stars, we have determined a temperature of 17,000+/-500 K. The resulting Cepheid mass is 6.0+/-0.4 Msolar. This mass is consistent with main-sequence evolutionary tracks with a moderate amount of convective overshoot.Based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer satellite. FUSE is operated for NASA by Johns Hopkins University, under NASA contract NAS5-32985.