We have discovered a highly significant overdensity of galaxies at z = 2.300 c 0.015 in the course of a redshift survey designed to select star-forming galaxies in the redshift range z = 2.3 c 0.4 in the field of the bright z = 2.72 QSO HS 1700+643. The structure has a redshift-space galaxy overdensity of d super(z) sub(g) 7 and an estimated matter overdensity in real space of d sub(m) 1.8, indicating that it will virialize by z 6 0 with a mass scale of 1.4 x 10 super(15) M sub( ), that of a rich galaxy cluster. Detailed modeling of the spectral energy distribution--from the rest-frame far-UV to the rest-frame near-IR--of the 72 spectroscopically confirmed galaxies in this field for which we have obtained K sub(s) and Spitzer IRAC photometry allows for a first direct comparison of galaxy properties as a function of large-scale environment at high redshift. We find that galaxies in the protocluster environment have mean stellar masses and inferred ages that are 62 times larger (at z = 2.30) than identically UV-selected galaxies outside of the structure, and we show that this is consistent with simple theoretical expectations for the acceleration of structure formation in a region that is overdense on large scales by the observed amount. The protocluster environment contains a significant number of galaxies that already appear old, with large stellar masses (>10 super(11) M sub( )), by z = 2.3.