OGLE-2017-BLG-0173L is a Sun-like G-type star (about 0.40 solar masses, 5,778 K), 4,705 light-years from Earth. OGLE-2017-BLG-0173L b is a 3.3-Earth-mass planet, at 3.913 AU from its star, discovered in 2018 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. OGLE-2017-BLG-0173L b circles in the outer system, comparable to the Jupiter region. Its low mass makes OGLE-2017-BLG-0173L b a Neptune- or super-Earth-class world rather than a gas giant.