  We report the discovery of an ultra-faint Milky Way satellite galaxy in the
constellation of Pegasus. The concentration of stars was detected by applying
our overdensity detection algorithm to the SDSS-DR 10 and confirmed with deeper
photometry from the Dark Energy Camera at the 4-m Blanco telescope. Fitting
model isochrones indicates that this object, Pegasus III, features an old and
metal-poor stellar population ([Fe/H]$\sim-2.1$) at a heliocentric distance of
$205\pm20$ kpc. The new stellar system has an estimated half-light radius of
$r_h=78^{+30}_{-24}$ pc and a total luminosity of $M_{V}\sim-4.1\pm0.5$ that
places it into the domain of dwarf galaxies on the size--luminosity plane.
Pegasus III is spatially close to the MW satellite Pisces II. It is possible
that the two might be physically associated, similar to the Leo IV and Leo V
pair. Pegasus III is also well aligned with the Vast Polar Structure, which
suggests a possible physical association.
