  We present the discovery of a rapidly evolving transient by the Korean
Microlensing Telescope Network Supernova Program (KSP). KSP is a novel
high-cadence supernova survey that offers deep ($\sim21.5$ mag in $BVI$ bands)
nearly continuous wide-field monitoring for the discovery of early and/or fast
optical transients. KSP-OT-201509a, reported here, was discovered on 2015
September 27 during the KSP commissioning run in the direction of the nearby
galaxy NGC~300, and stayed above detection limit for $\sim$ 22 days. We use our
$BVI$ light-curves to constrain the ascent rate, $-3.7(7)$ mag day$^{-1}$ in
$V$, decay time scale, $t^{V}_{2}=1.7(6)$ days, and peak absolute magnitude,
$-9.65\leq M_{V}\leq -9.25$ mag. We also find evidence for a short-lived
pre-maximum halt in all bands. The peak luminosity and lightcurve evolution
make KSP-OT-201509a consistent with a bright, rapidly decaying nova outburst.
We discuss constraints on the nature of the progenitor and its environment
using archival HST/ACS images and conclude with a broad discussion on the
nature of the system.
