  We report the detection of maser emission from the $J=4-3$ transition of
HC$_3$N at 36.4~GHz towards the nearby starburst galaxy NGC253. This is the
first detection of maser emission from this transition in either a Galactic or
extragalactic source. The HC$_3$N maser emission has a brightness temperature
in excess of 2500 K and is offset from the center of the galaxy by
approximately 18 arcsec (300 pc), but close to a previously reported class~I
methanol maser. Both the HC$_3$N and methanol masers appear to arise near the
interface between the galactic bar and the central molecular zone, where it is
thought that molecular gas is being transported inwards, producing a region of
extensive low-velocity shocks.
