TOI-270 is a cool M-dwarf (about 0.39 solar masses, 3,506 K, 0.38 solar radii), 22.5 light-years from Earth. TOI-270 is a nearby benchmark system: one super-Earth and two sub-Neptunes suited to comparative atmospheric study. TOI-270 b is a 1.5-Earth-mass planet, with an orbital period of 3.4 days, and an equilibrium temperature near 600 K, discovered in 2019 via transit. TOI-270 c is a 6.2-Earth-mass planet, with an orbital period of 5.7 days, and an equilibrium temperature near 489 K, discovered in 2019 via transit. TOI-270 d is a 4.2-Earth-mass planet, with an orbital period of 11.4 days, and an equilibrium temperature near 383 K, discovered in 2019 via transit.