  We present the 2020 version of the Siena Galaxy Atlas (SGA-2020), a
multi-wavelength optical and infrared imaging atlas of 383,620 nearby galaxies.
The SGA-2020 uses optical $grz$ imaging over $\approx20,000$ deg$^{2}$ from the
DESI Legacy Imaging Surveys Data Release 9 and infrared imaging in four bands
(spanning 3.4-22 $\mu$m) from the six-year unWISE coadds; it is more than 95%
complete for galaxies larger than $R(26)\approx25$ arcsec and $r<18$ measured
at the 26 mag arcsec$^{-2}$ isophote in the $r$-band. The atlas delivers
precise coordinates, multi-wavelength mosaics, azimuthally averaged optical
surface brightness profiles, model images and photometry, and additional
ancillary metadata for the full sample. Coupled with existing and forthcoming
optical spectroscopy from the Dark Energy Spectroscopic Instrument (DESI), the
SGA-2020 will facilitate new detailed studies of the star formation and mass
assembly histories of nearby galaxies; enable precise measurements of the local
velocity field via the Tully-Fisher and Fundamental Plane relations; serve as a
reference sample of lasting legacy value for time-domain and multi-messenger
astronomical events; and more.
