  We report the discovery of HAT-P-54b, a planet transiting a late K dwarf star
in field 0 of the NASA K2 mission. We combine ground-based photometric light
curves with radial velocity measurements to determine the physical parameters
of the system. HAT-P-54b has a mass of 0.760 $\pm$ 0.032 $M_J$, a radius of
0.944 $\pm$ 0.028 $R_J$, and an orbital period of 3.7998 d. The star has V =
13.505 $\pm$ 0.060, a mass of 0.645 $\pm$ 0.020 $M_{\odot}$, a radius of 0.617
$\pm$ 0.013 $R_{\odot}$, an effective temperature of Teff = 4390 $\pm$ 50K, and
a subsolar metallicity of [Fe/H] = -0.127 $\pm$ 0.080. HAT-P-54b has a radius
that is smaller than 92% of the known transiting planets with masses greater
than that of Saturn, while HAT-P-54 is one of the lowest-mass stars known to
host a hot Jupiter. Follow-up high-precision photometric observations by the K2
mission promise to make this a well-studied planetary system.
