The GAT-X131 Calamity Gundam is a Mobile Suit introduced in the anime Mobile Suit Gundam SEED. It is piloted by "Biological CPU" Orga Sabnak.

One of three second-generation GAT-X machines, Calamity is built on the X100-series frame and inherits GAT-X103 Buster Gundam's artillery attack concept. It can also be used to provide support from the rear and is armed with several powerful range weapons, but it completely lacks any close combat weaponry. Its armament consisted of two back-mounted high-energy long-range beam cannons, a chest-mounted multi-phase energy cannon, 2-barrel ram cannon equipped shield attached to the left arm and a hand-carried plasma-sabot bazooka.

Due to its Trans-Phase armor, a more energy-efficient form of the Phase Shift armor, Calamity has a longer operational time than the first generation GAT-X machines and also has more energy for its power-hungry beam weapons. With the powerful thrusters installed around the body, it can hover over deserts or bodies of water easily. However, as it lacks true flight capability within the atmosphere, it often has to ride on top of GAT-X370 Raider Gundam when the latter is in MA mode to keep up with its flight-capable sister units when moving to the combat zone. In this configuration, Calamity uses Raider much like ZAFT suits use the Guul subflight lifter. The control system of the Calamity is very complex and its customized OS can only be used by the "Biological CPU".