K2-9 is a cool M-dwarf (about 0.30 solar masses, 3,390 K, 0.31 solar radii), 83.0 light-years from Earth. K2-9 b is a planet with 2.3 Earth radii, with an orbital period of 18.4 days, at 0.091 AU from its star, and an equilibrium temperature near 314 K, discovered in 2015 via transit. At least one planet may receive an Earth-like flux of starlight, placing this system on the short list of potentially habitable worlds worth detailed atmospheric study. K2-9 b orbits well inside Earth's distance from the Sun.