KMT-2018-BLG-1988L is a Sun-like G-type star (about 0.47 solar masses, 5,778 K), 4,150 light-years from Earth. KMT-2018-BLG-1988L b is a 5.6-Earth-mass planet, at 3.090 AU from its star, discovered in 2022 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-2018-BLG-1988L b circles in the outer system, comparable to the Jupiter region. Its low mass makes KMT-2018-BLG-1988L b a Neptune- or super-Earth-class world rather than a gas giant.