The GAT-X252 Forbidden Gundam is a Mobile Suit introduced in the anime Mobile Suit Gundam SEED. It is piloted by "Biological CPU" Shani Andras.

A second generation GAT-X machine, the Forbidden is designed for infiltrating and raiding enemy bases and to fulfil this role, it can transform into a "High-Speed Assault Form" by flipping its huge backpack unit over its head. Like the other two second generation units, the Forbidden is equipped with Trans-Phase armor (TP armor). Its armaments consists of head-mounted CIWS, forearm-mounted machine guns, a handheld scythe and backpack-mounted railguns and plasma cannon, both of which are usable only in "High-Speed Assault Form".

As indicated by its model code, the Forbidden utilizes a X200-series frame like the GAT-X207 Blitz Gundam, and as such it is also equipped with special equipment. One special equipment is the "Geschmeidig Panzer" energy deflection armor installed in its two massive shields. This equipment can deflect any range beam attacks that hits it, making the Forbidden virtually invulnerable to these attacks. Coupled with the fact that the shields themselves also have TP armor, the Forbidden is thus well protected from most beam and physical attacks. In addition, the "Geschmeidig Panzer" also allows the Forbidden to move underwater with ease, even though it is not specifically designed for aquatic use.

Another special equipment is the magnetic field generators installed on the railguns, they allow the bending of the plasma cannon's beam, making the attack harder to evade. Due to the powerful thrusters in the backpack, which is actually a specialized flight unit, the Forbidden can fly in the atmosphere and is one of the rare mobile suits to be capable of both aerial and aquatic combat without any additional equipment.