KMT-2019-BLG-0842L is a Sun-like G-type star (about 0.76 solar masses, 5,778 K), 3,320 light-years from Earth. KMT-2019-BLG-0842L b is a 10.3-Earth-mass planet, at 3.310 AU from its star, discovered in 2020 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-2019-BLG-0842L b circles in the outer system, comparable to the Jupiter region. Its low mass makes KMT-2019-BLG-0842L b a Neptune- or super-Earth-class world rather than a gas giant.