We report the discovery of an ultra-compact dwarf (UCD) associated with the Sombrero galaxy (M104). This is the closest massive UCD known and the first spectroscopically verified massive UCD which is located in a low-density environment. The object, we name SUCD1, was identified in Hubble Space Telescope (HST)/Advanced Camera for Surveys (ACS) imaging and confirmed to be associated with the Sombrero galaxy by its recession velocity obtained from Keck spectra. The light profile is well fitted by a Wilson model. We measure a half-light size of 14.7 ± 1.4 pc , an absolute magnitude of MV=−12.3 mag (MK=−15.1 mag) and an internal velocity dispersion of 25.0 ± 5.6 km s−1 . Such values are typical of UCDs. From Lick spectral indices we measure a luminosity-weighted central age of 12.6 ± 0.9 Gyr, [Fe/H] of −0.08 ± 0.08 dex and [α/Fe] of 0.06 ± 0.07 dex . The lack of colour gradients suggests these values are representative of the entire UCD. The derived stellar and virial masses are the same, within errors, at ∼3.3 x 107 M⊙ . Thus, we find no strong evidence for dark matter or the need to invoke a non-standard initial mass function. We also report arguably the first X-ray detection of a bona fide UCD, which we attribute to the presence of low-mass X-ray binaries. The X-ray luminosity of LX= 0.56 x 1038 erg s−1 is consistent with the values observed for globular clusters (GCs) of the same metallicity. Overall, we find SUCD1 has properties similar to other known UCDs and massive GCs.