We present the discovery of a massive, quiescent galaxy at z = 2.99. We have obtained an Hubble Space Telescope WFC3 spectrum of this object and measured its redshift from the detection of a deep 4000 Å break consistent with an old population and a high metallicity. By stellar population modeling of both its grism spectrum and broadband photometry, we derive an age of 0.7 Gyr, implying a formation redshift of z > 4, and a mass >1011 M Ȯ. Although this passive galaxy is the most distant confirmed so far, we find that it is slightly less compact than other z > 2 early types of similar mass, being overall more analogous to those z 1.6 field early-type galaxies. The discovery of this object shows that early-type galaxies are detectable to at least z = 3 and suggests that the diversity of structural properties found in z = 1.4-2 ellipticals to earlier epochs could have its origin in a variety of formation histories among their progenitors.