We report the discovery of a new ultra-short-period planet and summarize the properties of all such planets for which the mass and radius have been measured. The new planet, EPIC~228732031b, was discovered in {it K2} Campaign 10. It has a radius of 1.81$^{+0.16}_{-0.12}~R_{oplus}$ and orbits a G dwarf with a period of 8.9 hours. Radial velocities obtained with Magellan/PFS and TNG/HARPS-N show evidence for stellar activity along with orbital motion. We determined the planetary mass using two different methods: (1) the floating chunk offset method, based only on changes in velocity observed on the same night, and (2) a Gaussian process regression based on both the radial-velocity and photometric time series. The results are consistent and lead to a mass measurement of $6.5 pm 1.6~M_{oplus}$, and a mean density of $6.0^{+3.0}_{-2.7}$~g~cm$^{-3}$.
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