  We report the discovery of a luminous late-time source at the position of the
fast blue optical transient (FBOT) AT2018cow on images taken by the Hubble
Space Telescope (HST) at 714 d and 1136 d after its explosion. This source is
detected at both UV and optical wavelengths and has prominent H$\alpha$
emission. It has a very stable brightness between the two epochs and a very
blue spectral energy distribution (SED) consistent with $f_\lambda$ $\propto$
$\lambda^{-4.1 \pm 0.1}$, i.e. the Rayleigh-Jeans tail of a hot blackbody with
a very high temperature of log($T$/K) $>$ 4.6 and luminosity of
log($L$/$L_\odot$) $>$ 7.0. This late-time source is unlikely to be an
unrelated object in chance alignment, or due to a light echo of AT2018cow.
Other possible scenarios also have some difficulties in explaining this
late-time source, including companion star(s), star cluster, the survived
progenitor star, interaction with circumstellar medium (CSM), magnetar, or
delayed accretion in a tidal disruption event (TDE). Long-term and
multi-wavelength monitoring will help to resolve its nature and finally reveal
the origin of the "Cow".

High Energy Astrophysical Phenomena;Solar and Stellar Astrophysics