KMT-2025-BLG-0121L is a Sun-like G-type star (about 0.61 solar masses, 5,778 K), 6,200 light-years from Earth. KMT-2025-BLG-0121L b is a 7.77-Jupiter-mass companion, at 3.050 AU from its star, discovered in 2026 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-2025-BLG-0121L b circles in the outer system, comparable to the Jupiter region. At 7.8 Jupiter masses, the companion lies near the dividing line between giant planets and brown dwarfs.