We present multifrequency VLA observations, multicolor CCD imaging, optical spectroscopy, and ROSAT HRI observations of the giant FR II radio galaxy NVSS 2146+82. This galaxy, which was discovered by the NRAO VLA Sky Survey, has an angular extent of nearly 20' from lobe to lobe. The radio structure is normal for an FR II source except for its large size and regions in the lobes with unusually flat radio spectra. Our spectroscopy indicates that the optical counterpart of the radio core is at redshift z = 0.145, so the linear size of the radio structure is ~4 h Mpc, H0 = 50 h50 km s-1 Mpc-1. This object is therefore the second-largest FR II galaxy known (3C 236 is ~6 h Mpc). Optical imaging of the field surrounding the host galaxy reveals an excess of candidate galaxy cluster members above the number typically found in the field surrounding a giant radio galaxy. WIYN Hydra spectra of a sample of the candidate cluster members reveal that six share the same redshift as NVSS 2146+82, indicating the presence of at least a rich group containing the FR II host galaxy. ROSAT HRI observations of NVSS 2146+82 place upper limits on the X-ray flux of 1.33 × 10-13 ergs cm-2 s-1 for any hot intergalactic medium and 3.52 × 10-14 ergs cm-2 s-1 for an X-ray active galactic nucleus (AGN), thereby limiting any X-ray emission at the distance of the radio galaxy to that typical of a poor group or weak AGN. Several other giant radio galaxies have been found in regions with overdensities of nearby galaxies, and a separate study has shown that groups containing FR II galaxies are underluminous in X-rays compared with groups without radio sources. We speculate that the presence of the host galaxy in an optically rich group of galaxies that is underluminous in X-rays may be related to the giant radio galaxy phenomenon.