KMT-2021-BLG-2609L is a Sun-like G-type star (about 0.20 solar masses, 5,778 K), 7,480 light-years from Earth. KMT-2021-BLG-2609L b is a 10.2-Earth-mass planet, at 2.300 AU from its star, discovered in 2024 via microlensing. Microlensing detections occur when a foreground star's gravity briefly magnifies a background source, revealing planets that are otherwise too faint and distant to observe directly. KMT-2021-BLG-2609L b circles in the outer system, comparable to the Jupiter region. Its low mass makes KMT-2021-BLG-2609L b a Neptune- or super-Earth-class world rather than a gas giant.