The 2023 release of the XMM OM Serendipitous Ultraviolet Source Survey (XMM-SUSS6.1) Catalog, a catalog of optical/UV sources detected by the Optical Monitor (OM) on-board the European Space Agency's (ESA's) XMM-Newton observatory, spans the period of observations from 2000 to November 2022. The data processing was performed at the European Space Astronomy Centre (ESAC, Madrid, Spain) using the XMM Science Analysis Software system (SAS) versions 18 and 19. In addition to covering a larger observation period, this sixth release reflects a change in philosophy with regard to the origin of the incorporated data. In previous releases, the data were generated via a bespoke processing of the OM Observation Data Files (ODFs) while in this new release, the catalog has been guided by the XMM user community and the authors have sought to harmonize the contents of the catalog with those of the OM data in the XMM-Newton Science Archive (XSA), which derive from the standard XMM-Newton pipeline processing system. While the bespoke processing and pipeline systems are fundamentally very similar, they are not identical and the differences lead to some differences in the output. The number of observations (OBSIDs) included in the catalog is 12,057. The total number of entries in this release is 9,920,390. They correspond to 6,659,554 unique sources, of which 1,225,117 have multiple entries in the source table, corresponding to different observations. For each entry, positional and photometric data (count rate, magnitude and flux) and quality flags for each measurement are provided. The description of the previous release of the catalog can be found in Page M.J. et al. (2012, MNRAS, 426, 903). U, B, V, UVW2, UVM2 and UVW1 refer to the filter bandpasses defined in the Source Properties: Filter Set section of the MSSL documentation for this catalog: <a href="http://www.mssl.ucl.ac.uk/www_astro/XMM-OM-SUSS/SourcePropertiesFilters.shtml">http://www.mssl.ucl.ac.uk/www_astro/XMM-OM-SUSS/SourcePropertiesFilters.shtml</a>;. There is a second, related table which gives a summary of the observations from which the XMM-SUSS6.1 sources listed in this table have been detected and measured. That summary table is available at the HEASARC as the <a href="/W3Browse/xmm-newton/xmmomsuob.html">XMMOMSUOB table</a>. This HEASARC database table contains the sixth release of the XMM-OM SUSS catalog, XMM-SUSS6.1, released by ESA in October 2023, obtained from the XMM-Newton Science Archive (<a href="http://xmm.esac.esa.int/xsa">http://xmm.esac.esa.int/xsa</a>;), and ingested into the HEASARC database in October 2023. It is also available at the HEASARC as the gzipped FITS file <a href="https://heasarc.gsfc.nasa.gov/FTP/xmm/data/catalogues/XMM-OM-SUSS6-1.1.fits.gz">https://heasarc.gsfc.nasa.gov/FTP/xmm/data/catalogues/XMM-OM-SUSS6-1.1.fits.gz</a>;. This is a service provided by NASA HEASARC .
