  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 (JADES), 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
$\sim$100 times above the empirical relation between infrared excess and UV
continuum slope (IRX-$\beta_\mathrm{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
$10^{10.8\pm0.1}\,\mathrm{M}_\odot$ and star-formation rate of
$10^{2.8\pm0.2}\,\mathrm{M}_\odot\,\mathrm{yr}^{-1}$, 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 $50\pm20$% of
the cosmic star formation at $z=5.1-5.5$ occur in protocluster environments.
