The F91 Gundam F91 (F91 ガンダムF91 Effu Kujūichi Gandamu Fōmyura Nainti Wan?) is a mobile suit introduced in the movie Mobile Suit Gundam F91, in which it was piloted by Seabook Arno. Harrison Madin piloted two production type units in custom colors.

Based on F90V Gundam F90 VSBR Type, the F91 is a prototype machine created under SNRI’s ‘Formula Project’. The Formula Project, also known as F Project, is a miniaturized MS development program meant to address technological problems such as the upsizing and increasing cost of MS, and to rectify Anaheim Electronics’ monopolization of the MS industry. The F91 was developed to reached the ‘performance limit tidemark of present day MS’, and as such, its capabilities exceed that of contemporary machines. SNRI only referred to it by its model number, F91. It was later given the code name "Gundam F91" by Space Ark captain Leahlee Edaberry as its face resembled the Gundam’s.

Befitting its smaller size, the Gundam F91 has a small reactor located in its enlarged backpack, surrounded by high-output thrusters. Despite its reduced size, the reactor has a high output, allowing the suit to drive multiple beam weapons including a beam rifle, two beam sabers, a beam launcher, a beam shield, and a pair of variable speed beam rifles (also known as VSBRs). While the first two beam weapons are common among contemporary mobile suits, the last three are of new designs. The beam launcher is a beam weapon equivalent of a projectile firing bazooka. The beam shield generates a highly defensive energy shield to block beam and projectile attacks. The VSBR is the Gundam F91’s most powerful weaponry, and is capable of firing beams of different speed to achieve either high destructive power or high penetrative power. Other weapons include two Vulcan guns and two mega machine cannons.

With its reduced size and the increase in output, the Gundam F91 has a drastically improved power-to-weight ratio, allowing it to attain higher mobility than MS of earlier eras. Its mobility is on par with the miniaturized MS deployed later by the Crossbone Vanguard. It also has special features such as the Bio-Computer, MCA (Multiple Construction Armor) structure, and the MEPE (Metal Peel-off Effect) phenomenon. The Bio-Computer is installed in the suit’s head and feeds battle data directly to the pilot, but only a Newtype pilot can draw out its full potential. Accompanying the installment of the Bio-Computer, a head-mounted coolant and heat discharging system were also implemented, with the face guard opening to discharge heat under certain circumstance.The MCA structure incorporates electrical functions into the suit’s armor, allowing for more efficient use of the F91's limited internal space. The MEPE is a byproduct of the MCA structure. It helps the suit to dissipate excess heat by shedding the outer layers of its armor, and in the process, creates afterimages with mass to confuse enemy pilots, as well as the sensors of enemy machines. However, the MEPE is only activated under specific conditions.

The Gundam F91’s high performance is not without drawbacks; its full operating mode is too much for normal pilots to handle. Thus, limiters were installed to make the suit more manageable; only a Newtype pilot can handle the full operating mode. When the Bio-Computer - via the Bio-Sensor in the pilot’s linear seat - determines that a Newtype is at the controls, it will disable the limiters and allow activation of the full operating mode. In this mode, the suit performs at its maximum potential and forced cooling is activated. This results in the suit radiating heat from all over its body: the face guard opens to vent heat, the shoulder-mounted heat radiations fins are deployed, the retractable thruster units on the back of the lower legs and the associated fins are activated, and the MEPE phenomenon occurs. All these are to reduce damage to the machine caused by movement overload during the full operating mode, and to protect the Bio-Computer that is sensitive to high heat. It is also worth noting that the front grill that takes up most of the chest and abdomen is a heat sink.

After the war with the Crossbone Vanguard, an unspecified but extremely small numbers of Gundam F91s were manufactured. The MCA, MEPE, and full spec Bio-Computer were omitted from these production type units; inclusion of these technologies in the prototype unit was more for experimental purposes. Despite this, the catalog specifications of these machines are still comparable to the prototype’s. Although the face guard opening mechanism is retained in all production type units, only certain units for ace pilots (such as Harrison Madin’s) can access the full operating mode through improvements in the cooling system and various devices. However, these ace-used units still do not have the MEPE function due to the enhanced cooling system, as well as the change in the materials used in the final layer of armor.