  We report the discovery of a new faint dwarf galaxy, which we dub Scl-MM-Dw1,
at a projected distance of $\sim$65 kpc from the spiral galaxy NGC 253. The
discovery results from the Panoramic Imaging Survey of Centaurus and Sculptor
(PISCeS), a program with the Magellan/Megacam imager to study faint
substructure in resolved stellar light around massive galaxies outside of the
Local Group. We measure a tip of the red giant branch distance to Scl-MM-Dw1 of
$D$=3.9$\pm$0.5 Mpc, consistent with that of NGC 253, making their association
likely. The new dwarf's stellar population is complex, with an old, metal poor
red giant branch ($\gtrsim$10 Gyr, [Fe/H]$\sim$$-$2), and an asymptotic giant
branch with an age of $\sim$500 Myr. Scl-MM-Dw1 has a half-light radius of
$r_{h}$=340$\pm$50 pc and an absolute magnitude of $M_{V}$=$-$10.3$\pm$0.6 mag,
comparable to the Milky Way's satellites at the same luminosity. Once complete,
our imaging survey of NGC 253 and other nearby massive galaxies will provide a
census of faint substructure in halos beyond the Local Group, both to put our
own environment into context and to confront models of hierarchical structure
formation.
