The ZGMF-X31S Abyss Gundam is a Mobile Suit introduced in the anime Mobile Suit Gundam SEED Destiny. Known pilots include Mare Strode and Auel Neider.

The Abyss Gundam is a prototype transformable amphibious mobile suit developed as part of ZAFT's Second Stage Series. It is designed to use its MS mode for combat in space and on the ground, and can transform into a specialized MA mode for underwater combat. The MA mode adopts a streamline-shape that helps to reduce water resistance, making the suit faster and more mobile in the water, and also grants it higher underwater combat performance than the other ZAFT amphibious mobile suits. Like the other units in the Second Stage Series, the Abyss Gundam has a new form of Phase Shift armor known as Variable Phase Shift armor and is powered by a battery that can be recharged during battle using the Deuterion Beam Energy Transfer System.

The Abyss is heavily armed and features a large number of beam and shell based weaponry, but most of them are usable only in MS mode. The MA mode relies mainly on the high-speed guided torpedos and dual cannons for underwater attacks, and the dual beam cannon for destroying targets on and above the water surface. The suit's beam weaponry are never used underwater as their power are reduced by the water. The Abyss Gundam can also use a handheld beam lance for close combat (even in the water as the lance has a physical blade), and for defense, it can use the pair of shoulder-mounted binders as shields as they are anti-beam coated. The suit mainly functions as a long range support unit due to its large array of ranged weaponry.