We report the discovery of an extraordinarily massive young cluster of stars in the Galaxy, having an inferred total initial cluster mass comparable to the most massive young clusters in the Galaxy. Using IRMOS, 2MASS, and Spitzer observations, we conclude that there are 14 red supergiants in the cluster, compared with five, in what was previously thought to be the richest Galactic cluster of such stars. We infer spectral types from near-infrared spectra that reveal deep CO bandhead absorption that can only be fit by red supergiants. We identify a gap of ΔKs~4 mag between the stars and the bulk of the other stars in the region that can only be fit by models if the brightest stars in the cluster are red supergiants. We estimate a distance of 5.8 kpc to the cluster by associating an OH maser with the envelope of one of the stars. We also identify a ``yellow'' supergiant of G6 I type in the cluster. Assuming a Salpeter IMF, we infer an initial cluster mass of 20,000-40,000 Msolar for cluster ages of 7-12 Myr. Continuing with these assumptions, we find that 80% of the initial mass and 99% of the number of stars remain at the present time. We associate the cluster with an X-ray source (detected by ASCA and Einstein), a recently discovered very high energy γ-ray source (detected by INTEGRAL and HESS), and several nonthermal radio sources, finding that these objects are likely related to recent supernovae in the cluster. In particular, we claim that the cluster has produced at least one recent supernova remnant with properties similar to the Crab Nebula.