We report the spectroscopic confirmation of a new protocluster in the COSMOS field at $z$ $sim$ 2.2, COSMOS Cluster 2.2 (CC2.2), originally identified as an overdensity of narrowband selected H$alpha$ emitting candidates. With only two masks of Keck/MOSFIRE near-IR spectroscopy in both $H$ ($sim$ 1.47-1.81 $mu$m) and $K$ ($sim$ 1.92-2.40 $mu$m) bands ($sim$ 1.5 hour each), we confirm 35 unique protocluster members with at least two emission lines detected with S/N $>$ 3. Combined with 12 extra members from the zCOSMOS-deep spectroscopic survey (47 in total), we estimate a mean redshift and a line-of-sight velocity dispersion of $z_{mean}$=2.23224 $pm$ 0.00101 and $sigma_{los}$=645 $pm$ 69 km s$^{-1}$ for this protocluster, respectively. Assuming virialization and spherical symmetry for the system, we estimate a total mass of $M_{vir}$ $sim$ $(1-2) times$10$^{14}$ $M_{odot}$ for the structure. We evaluate a number density enhancement of $delta_{g}$ $sim$ 7 for this system and we argue that the structure is likely not fully virialized at $z$ $sim$ 2.2. However, in a spherical collapse model, $delta_{g}$ is expected to grow to a linear matter enhancement of $sim$ 1.9 by $z$=0, exceeding the collapse threshold of 1.69, and leading to a fully collapsed and virialized Coma-type structure with a total mass of $M_{dyn}$($z$=0) $sim$ 9.2$times$10$^{14}$ $M_{odot}$ by now. This observationally efficient confirmation suggests that large narrowband emission-line galaxy surveys, when combined with ancillary photometric data, can be used to effectively trace the large-scale structure and protoclusters at a time when they are mostly dominated by star-forming galaxies.
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