We present the first spectroscopic confirmation of an ultramassive galaxy at redshift z\gt 3 using data from Keck-NIRSPEC, VLT-X-shooter, and GTC-Osiris. We detect strong [O iii] and Lyα emission, and weak [O ii], C iv, and He ii, placing C1-23152 at a spectroscopic redshift of {{z}spec}=3.351. The modeling of the emission-line-corrected spectral energy distribution (SED) results in a best-fit stellar mass of {{M}*}=3.1-0.7+0.6× {{10}11} {{M}ȯ }, a star formation rate of <7 {{M}ȯ } yr‑1, and negligible dust extinction. The stars 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 24 μm, we confirm the presence of a luminous hidden active galactic nucleus (AGN), with bolometric luminosity of ∼ {{10}46} erg s‑1. Potential contamination of the observed SED from the AGN continuum is constrained, placing a lower limit on the stellar mass of 2× {{10}11} {{M}ȯ }. 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 Sérsic 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\gt 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.