We report on the discovery of K2-141 b (EPIC 246393474 b), an ultra-short-period super-Earth on a 6.7 h orbit transiting an active K7 V star based on data from K2 campaign 12. We confirmed the planet’s 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 ± 0.46 M_⊕ and radius of 1.54_−0.09^+0.10 R_⊕, yielding a mean density of 8.00_−1.45^+1.83 g cm⁻³ and suggesting a rocky-iron composition. Models indicate that iron cannot exceed ~70% of the total mass. With an orbital period of only 6.7 h, K2-141 b is the shortest-period planet known to date with a precisely determined mass.