TOI-5800 is an orange K-type dwarf (about 0.78 solar masses, 4,850 K, 0.77 solar radii), 42.9 light-years from Earth. TOI-5800 b is a 9.5-Earth-mass planet, with an orbital period of 2.6 days, at 0.034 AU from its star, and an equilibrium temperature near 1,108 K, on an eccentric orbit (e ≈ 0.30), 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. The planet's marked eccentricity suggests past scattering events or perturbations from unseen companions.