  We report the first results from the Z-FOURGE survey: the discovery of a
candidate galaxy cluster at z = 2.2 consisting of two compact overdensities
with red galaxies detected at >20{\sigma} above the mean surface density. The
discovery was made possible by a new deep (Ks < 24.8 AB 5{\sigma})
Magellan/FourStar near-IR imaging survey with 5 custom medium-bandwidth
filters. The filters pinpoint the location of the Balmer/4000\AA\ break in
evolved stellar populations at 1.5 < z < 3.5, yielding significantly more
accurate photometric redshifts than possible with broadband imaging alone. The
overdensities are within 1' of each other in the COSMOS field and appear to be
embedded in a larger structure that contains at least one additional
overdensity (\sim10{\sigma}). Considering the global properties of the
overdensities, the z = 2.2 system appears to be the most distant example of a
galaxy cluster with a population of red galaxies. A comparison to a large
{\Lambda}CDM simulation suggests that the system may consist of merging
subclusters, with properties in between those of z > 2 protoclusters with more
diffuse distributions of blue galaxies and the lower-redshift galaxy clusters
with prominent red sequences. The structure is completely absent in public
optical catalogs in COSMOS and only weakly visible in a shallower near-IR
survey. The discovery showcases the potential of deep near-IR surveys with
medium-band filters to advance the understanding of environment and galaxy
evolution at z > 1.5.
