We present the discovery and confirmation of the ultra-short period (USP) planet TOI-2431 b orbiting a nearby (d~36pc) late K star (Teff=4109+/-28K) using observations from the Transiting Exoplanet Survey Satellite (TESS), precise radial velocities with the NEID and the Habitable-zone Planet Finder (HPF) spectrographs, as well as ground-based high contrast imaging from NESSI. TOI-2431 b has a period of 5 hours and 22 minutes, making it one of the shortest-period exoplanets known to date. TOI-2431 b has a radius of 1.536+/-0.033Re, and a mass of 6.2+/-1.2Me, suggesting it has a density compatible with an Earth-like composition and, due to its high irradiation, is likely a 'lava-world' with a Teq=2063+/-30K. We estimate that the current orbital period is only 30% larger than the Roche-limit orbital period, and that it has an expected orbital decay timescale of only ~31Myr. Finally, due to the brightness of the host star (V=10.9, K=7.6), TOI-2431 b has a high Emission Spectroscopy Metric of 27, making it one of the best USP systems for atmospheric phase-curve analysis.