We report on the discovery of K2-141 b (EPIC 246393474 b), an ultra-short-period super-Earth on a 6.7-hour orbit transiting an active K7 V star based on data from K2 campaign 12. We confirmed the planets existence and measured its mass with a series of follow-up observations: seeing-limited MuSCAT imaging, NESSI high-resolution speckle observations, and FIES and HARPS high-precision radial-velocity monitoring. K2-141 b has a mass of $5.31 pm 0.46 $ $M_{oplus}$ and radius of $1.54^{+0.10}_{-0.09}$ $R_{oplus}$, yielding a mean density of $8.00_{ - 1.45 } ^ { + 1.83 }$ $mathrm{g,cm^{-3}}$ and suggesting a rocky-iron composition. Models indicate that iron cannot exceed $sim$70 % of the total mass. With an orbital period of only 6.7 hours, K2-141 b is the shortest-period planet known to date with a precisely determined mass.
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