  We present the discovery of a massive, quiescent galaxy at z=2.99. We have
obtained a HST/WFC3 spectrum of this object and measured its redshift from the
detection of a deep 4000A break consistent with an old population and a high
metallicity. By stellar population modeling of both its grism spectrum and
broad-band photometry, we derive an age of ~0.7 Gyr, implying a formation
redshift of z>4, and a mass >10^11 Msun. 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.
