Large nebulae ( 50 kpc) emitting strongly in Ly? (also known as Ly? blobs) are likely signposts of ongoing massive galaxy formation. The relative rarity of these sources and their discovery in well-studied galaxy overdensities suggest that they may be associated with regions of high galaxy density. One of the largest Ly? nebulae, discovered at a redshift of -->z ? 2.7 via its strong mid-infrared emission, provides an unbiased test of this association. We have carried out a deep intermediate-band imaging survey for Ly?-emitting galaxies (LAEs) within a 30' --> ? 26' --> field of view around this Ly? nebula. This is the first study of the environment of a Ly? nebula found without a priori knowledge of its surroundings. We find that the nebula is located in an overdense region, at least 20 ? 50 -->h70?1 comoving Mpc in size, showing a factor of ~3 LAE number density enhancement relative to the edge of the field. Given the predicted number of such overdensities, we rule out the possibility of a chance coincidence at the 1% level. This study, in conjunction with previous work, provides strong confirmation of the association between the largest Ly? nebulae and overdense regions of the universe.