Spectra of SN 1993J at three epochs within the first 1.5 months after the explosion reveal the onset of a remarkable transformation. The prominent Hα emission line visible at early times became progressively weaker as P Cygni features of He I gradually appeared; SN 1993J is therefore a hydrogen-poor, helium-rich Type IIb supernova. The progenitor of SN 1993J was probably a reasonably massive star (initial mass 10-17 M ○. ) that transferred most, but not all, of its hydrogen envelope to a physically bound companion. We predict that many (≥6) months past maximum brightness, the spectrum of SN 1993J will closely resemble the late-time spectra of Type Ib supernovae; it will be dominated by strong emission lines of [O I], [Ca II], and Ca II, with Hα very weak or absent