We report the spectroscopic confirmation of a new protocluster in the COSMOS field at z ∼ 2.2, originally identified as an overdensity of narrow-band selected Hα emitting candidates. With only two masks of Keck/MOSFIRE near-IR spectroscopy in both H (∼ 1.47-1.81 μm) and K (∼ 1.92- 2.40 μm) bands (∼ 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, line-of-sight velocity dispersion, and total mass of zmean=2.23224 ± 0.00101, σlos=645 ± 69 km s−1, and Mvir ∼ (1 − 2)×10^14 M⊙ for this protocluster, respectively. We estimate a number density enhancement of δg ∼ 7 for this system and we argue that the structure is likely not virialized at z ∼ 2.2. However, in a spherical collapse model, δg is expected to grow to a linear matter enhancement of ∼ 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 Mdyn(z=0) ∼ 9.2×10^14 M⊙ by now. This observationally efficient confirmation suggests that large narrow-band 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.