HDF850.1 is the brightest submillimeter galaxy (SMG) in the Hubble Deep Field. It is known as a heavily dust-obscured star-forming galaxy embedded in an overdense environment at z=5.18. With nine-band NIRCam images at 0.8-5.0{mu}m obtained through the JWST Advanced Deep Extragalactic Survey, we detect and resolve the rest-frame UV-optical counterpart of HDF850.1, which splits into two components because of heavy dust obscuration in the center. The southern component leaks UV and H{alpha} photons, bringing the galaxy ~100 times above the empirical relation between infrared excess and UV continuum slope (IRX-{beta}UV). The northern component is higher in dust attenuation and thus fainter in UV and H{alpha} surface brightness. We construct a spatially resolved dust-attenuation map from the NIRCam images, well matched with the dust continuum emission obtained through millimeter interferometry. The whole system hosts a stellar mass of 10^10.8{\pm}0.1^M_{sun} and star formation rate (SFR) of 10^2.8{\pm}0.2^M{sun}_/yr, placing the galaxy at the massive end of the star-forming main sequence at this epoch. We further confirm that HDF850.1 resides in a complex overdense environment at z=5.17-5.30, which hosts another luminous SMG at z=5.30 (GN10). The filamentary structures of the overdensity are characterized by 109 H{alpha}-emitting galaxies confirmed through NIRCam slitless spectroscopy at 3.9-5{mu}m, of which only eight were known before the JWST observations. Given the existence of a similar galaxy overdensity in the GOODS-S field, our results suggest that 50%{\pm}20% of the cosmic star formation at z=5.1-5.5 occur in protocluster environments.

Cone search capability for table J/ApJ/961/69/table2 (*Summary of spectroscopically confirmed H{alpha}-emitting galaxies at z=5.1-5.5 (see Appendix))