TOI-4666 is a cool M-dwarf (about 0.58 solar masses, 3,793 K, 0.58 solar radii), 155 light-years from Earth. TOI-4666 b is a 0.49-Jupiter-mass companion, with an orbital period of 2.9 days, at 0.034 AU from its star, and an equilibrium temperature near 695 K, discovered in 2025 via transit. The inner world is a hot Jupiter — a gas giant hugging its star at a fraction of Mercury's distance — a class that forced astronomers to rethink how giant planets form and migrate inward. TOI-4666 b skims its star closer than Mercury orbits the Sun.