We are on the cusp of a revolution in massive star research triggered by 2MASS and Spitzer/GLIMPSE, and now is the ideal time to capitalize on these projects by performing the first survey of massive stars in young stellar clusters throughout the Galactic plane. A search of the 2MASS survey has produced over 450 newly-identified massive stellar cluster candidates which are hidden from our view at optical wavelengths due to extinction. We leverage the capabilities of Spitzer/IRAC to find and measure physical properties of the most massive stars in the best 86 targets in this sample. We propose a small program of 11 hours to image the candidate clusters in all four IRAC channels using a GLIMPSE-like observing strategy. With an analysis of these data, and those already existing through GLIMPSE, we expect to double the number of massive stars known in the Galaxy. This program addresses fundamental questions whose answers are basic requirements for studying many of the most important topics in Astrophysics: the slope to the initial mass function (IMF), an upper mass cutoff to the IMF, the formation and evolution of the most massive stars, the effects of massive stars on lower mass protostellar/protoplanetary systems, gamma-ray burst (GRB) progenitors, nature of the first stars in the Universe, chemical enrichment of the interstellar medium, Galactic gas dynamics, star formation in starbursts and merging galaxies (particularly in the early Universe). The proposed program is timely in that the young cluster targets have only recently been discovered and are going to be the subject of intense observations with HST and Chandra, as well as numerous approved and planned ground-based spectroscopic and radio observations. We have assembled an experienced team who have previously performed similar studies on much smaller samples, including the massive young clusters in the Galactic center, the Arches and Quintuplet cluster.