We present the Siena Galaxy Atlas 2020 (SGA-2020), a multiwavelength atlas of approximately 400,000 nearby galaxies constructed from new deep, wide-area imaging from the DESI Legacy Imaging Surveys (20,000 sq. degrees in grz to r<23.4) and all-sky infrared imaging at 3.4-22 microns from WISE. The atlas delivers precise coordinates, multiwavelength mosaics, azimuthally averaged surface brightness profiles, integrated and aperture photometry, and additional metadata for the full sample. The SGA-2020 will yield new insights into the history of galaxy formation through detailed studies of the internal structure, global physical properties, environments, and faint, low surface-brightness features of galaxies, and will support current and future cosmological surveys like DESI by enabling precise masking of large angular diameter foreground galaxies. We describe the input imaging data, the selection of the sample primarily by apparent diameter, the construction of the atlas, and the release of the SGA-2020 as a supplement to the ninth public data release (DR9) of the DESI Legacy Imaging Surveys through legacysurvey.org.