Procyon B is one of a select group of white dwarf stars in visual binaries that are used to put stringent constraints on the fundamental physics of stellar degeneracy. We present Space Telescope Imaging Spectrograph observations of this elusive white dwarf. Our spectra cover 1800-10000 Å and contain carbon Swan bands, Mg II, and numerous iron features. Procyon B is therefore a rare DQZ white dwarf. Our analysis of the spectrum yields an effective temperature of 7740+/-50 K and a radius of 0.01234+/-0.00032 Rsolar. We discuss the implications of these results on observational support for the white dwarf mass-radius relation.