jats:titleAbstract</jats:title> jats:pHDF850.1 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 jats:italicz</jats:italic> = 5.18. With nine-band NIRCam images at 0.8–5.0 jats:italicμ</jats:italic>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 Hjats:italicα</jats:italic> photons, bringing the galaxy ∼100 times above the empirical relation between infrared excess and UV continuum slope (IRX–jats:italicβ</jats:italic> jats:subUV</jats:sub>). The northern component is higher in dust attenuation and thus fainter in UV and Hjats:italicα</jats:italic> 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 10jats:sup10.8±0.1</jats:sup> jats:italicM</jats:italic> jats:sub⊙</jats:sub> and star formation rate (SFR) of 10jats:sup2.8±0.2</jats:sup> jats:italicM</jats:italic> jats:sub⊙</jats:sub> yrjats:sup−1</jats:sup>, 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 jats:italicz</jats:italic> = 5.17–5.30, which hosts another luminous SMG at jats:italicz</jats:italic> = 5.30 (GN10). The filamentary structures of the overdensity are characterized by 109 Hjats:italicα</jats:italic>-emitting galaxies confirmed through NIRCam slitless spectroscopy at 3.9–5 jats:italicμ</jats:italic>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% ± 20% of the cosmic star formation at jats:italicz</jats:italic> = 5.1–5.5 occur in protocluster environments.</jats:p>