OGLE-2005-BLG-390L is a Sun-like G-type star (about 0.22 solar masses, 5,778 K), 6,600 light-years from Earth. OGLE-2005-BLG-390L b was the first cold, low-mass planet found by microlensing, a super-Earth far from its star. OGLE-2005-BLG-390L b is a 5.5-Earth-mass planet, with an orbital period of 8.99 years, at 2.600 AU from its star, and an equilibrium temperature near 50.0 K, discovered in 2005 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-2005-BLG-390L b circles in the outer system, comparable to the Jupiter region. Its low mass makes OGLE-2005-BLG-390L b a Neptune- or super-Earth-class world rather than a gas giant.