We present the first spectroscopic confirmation of an ultra-massive galaxy at redshift z>3 using data from Keck-NIRSPEC, VLT-Xshooter, and GTC-Osiris. We detect strong [OIII] and Lyα emission, and weak [OII], CIV, and HeII, placing C1-23152 at a spectroscopic redshift of z_spec=3.351. The modeling of the emission-line corrected spectral energy distribution results in a best-fit stellar mass of M_*=3.1^+0.6_-0.7×10^11 M_, a star-formation rate of <7 M_ yr^-1, and negligible dust extinction. The

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... tars appear to have formed in a short intense burst 300-500 Myr prior to the observation epoch, setting the formation redshift of this galaxy at z 4.1. From the analysis of the line ratios and widths, and the observed flux at 24μm, we confirm the presence of a luminous hidden active galactic nucleus (AGN), with bolometric luminosity of 10^46erg s^-1. Potential contamination to the observed SED from the AGN continuum is constrained, placing a lower limit on the stellar mass of 2×10^11 M_. HST/WFC3 H_160 and ACS I_814 images are modeled, resulting in an effective radius of r_e 1 kpc in the H_160 band and a Sersic index n 4.4. This object may be a prototype of the progenitors of local most massive elliptical galaxies in the first 2 Gyr of cosmic history, having formed most of its stars at z>4 in a highly dissipative, intense, and short burst of star formation. C1-23152 is completing its transition to a post-starburst phase while hosting a powerful AGN, potentially responsible for the quenching of the star formation activity.