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+/0.032M_J_, a radius of 0.944+/-0.028R_J_, and an orbital period of 3.7998days. The star has V=13.505+/0.060, a mass of 0.645+/-0.020M_{sun}, a radius of 0.617+/-0.013R{sun}, an effective temperature of T_eff*=4390+/-50, and a subsolar metallicity of [Fe/H]=-0.127+/-0.080. We also detect a periodic signal with P=15.6days and 5.6mmag amplitude in the light curve, which we interpret as due to the rotation of the star. 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.