TOI-6478 is a cool M-dwarf (about 0.23 solar masses, 3,230 K, 0.23 solar radii), 38.6 light-years from Earth. TOI-6478 b is a planet with 4.6 Earth radii, with an orbital period of 34.0 days, at 0.114 AU from its star, and an equilibrium temperature near 204 K, discovered in 2025 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-6478 b orbits well inside Earth's distance from the Sun.