TOI-1231 is a cool M-dwarf (about 0.48 solar masses, 3,553 K, 0.48 solar radii), 27.6 light-years from Earth. TOI-1231 b is a 15.4-Earth-mass planet, with an orbital period of 24.2 days, at 0.129 AU from its star, and an equilibrium temperature near 330 K, on an eccentric orbit (e ≈ 0.09), discovered in 2021 via transit. The planet was found when it crossed the face of its star, producing periodic dips in brightness that reveal its size and orbital period to space-based photometry. TOI-1231 b orbits well inside Earth's distance from the Sun.