The Australia Telescope 20-GHz Survey (AT20G) is a blind radio survey carried out at 20 GHz with the Australia Telescope Compact Array (ATCA) from 2004 to 2008, and covers the whole sky south of declination 0 degrees. The AT20G source catalogue presented here is an order of magnitude larger than any previous catalogue of high-frequency radio sources, and includes 5890 sources above a 20-GHz flux-density limit of 40 mJy. All AT20G sources have total intensity and polarisation measured at 20 GHz, and most sources south of declination -15 degrees also have near-simultaneous flux-density measurements at 5 and 8 GHz. A total of 1559 sources were detected in polarised total intensity at one or more of the three frequencies. The completeness of the AT20G source catalog is 91% above 100 mJy/beam and 79% above 50 mJy/beam in regions south of Declination -15 degrees. North of -15 degrees, some observations of sources between 14 and 20 hours in RA were lost due to bad weather and could not be repeated, so the catalog completeness is lower in this region. Each detected source was visually inspected as part of the authors' quality control processs, and so the reliability of the final catalog is essentially 100%. This table was created by the HEASARC in March 2010 based on the <a href="https://cdsarc.cds.unistra.fr/ftp/cats/J/MNRAS/402/2403/">CDS Catalog J/MNRAS/402/2403/</a> file at20gcat.dat. This is a service provided by NASA HEASARC .