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.
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