We report the discovery of possible overdensities of galaxies at z ~ 3.7 in Chandra Deep Field-South (CDF-S). These overdensities are identified from a photometric redshift-selected sample and a BVz-selected sample. One overdensity is identified in the proximity of two active galactic nuclei and Lyman break galaxies at z = 3.66 and z = 3.70 at 7σ significance level. The other overdensity is less significant. It is identified around six z spec 3.6 galaxies at 3σ significance level. The line-of-sight velocity dispersions of these overdensities are found to be σ v 500-800 km s–1, comparable to the velocity dispersions of clusters of galaxies today. Through spectral energy distribution fitting, we find ~15 massive galaxies with M 1011 M ☉ around the z 3.7 overdensity. The mass of the z 3.7 overdensity is found to be a few ×1014 M ☉. Our result suggests that high-redshift overdense regions can be found in a supposedly blank field, and that the emergence of massive structures can be traced back to redshifts as high as z ~ 3.7.