The P90 is a selective fire, straight blowback-operated weapon with a cyclic rate of fire of 900 RPM (rounds per minute). The weapon is chambered for FN's 5.7×28mm ammunition, and although it was developed and initially marketed as a personal defense weapon, it could be considered a submachine gun or compact assault rifle. Its unusual shape is based on extensive ergonomic research. The weapon is grasped by means of a thumbhole in the frame that acts as a pistol grip, as well as an oversized trigger guard that acts as a foregrip for the shooter's support hand. The P90 fires from a closed bolt for maximum accuracy, and its design makes extensive use of polymers for reduced weight and cost. Overall, the weapon is relatively lightweight, weighing 2.5 kg (5.6 lb) empty, or 3 kg (6.6 lb) with a loaded 50-round magazine.
The P90 is notable for being fully ambidextrous—it can be operated by right or left-handed shooters with equal ease, and without making any modifications to the weapon. FN Herstal has described it as the "first fully ambidextrous individual automatic weapon." The charging handle, magazine release and backup iron sights are symmetrically distributed on both sides of the weapon, and the firing selector is located directly at the foot of the trigger, where it can be operated from either side by the shooter's trigger finger or support hand thumb. When fired, the P90 ejects spent cartridge casings downward through a chute located behind the grip, so spent cases are kept out of the shooter's line of sight.
The P90 can be fitted with a sling for greater ease of carry, and since the weapon has a fixed stock (as opposed to having a collapsing or folding stock), it can be quickly deployed when needed. The weapon's smooth, rounded contours prevent it from snagging on the shooter's clothing or equipment, and a small vertical protrusion is provided at the front end of the weapon's frame to prevent the shooter's hand from accidentally slipping in front of the muzzle while shooting. A hollow compartment inside the rear of the weapon's frame—accessed by removing the buttplate—allows for convenient storage of a cleaning kit.
The P90 was designed to have a length no greater than a man's shoulder width, to allow it to be easily carried and maneuvered in tight spaces, such as the inside of an armored vehicle. To achieve this, the weapon's design uses the unconventional bullpup configuration, in which the action and magazine are located behind the trigger and alongside the shooter's face so that there is no wasted space in the stock. The P90's dimensions are minimized by its unique horizontally mounted feeding system, wherein the box magazine sits parallel to the barrel on top of the weapon's frame. The weapon overall has an extremely compact profile—it is the most compact fixed-stock submachine gun to be made. The standard version of the weapon has an overall length of 500 mm (19.7 in), a height of 210 mm (8.3 in), and a width of 55 mm (2.2 in).
The P90 requires minimal maintenance, and it can be disassembled quickly and easily. It is a modular firearm, consisting of four main component groups: the barrel and optical sight group, the moving parts group, the frame and trigger group, and the hammer group. The P90's barrel is cold hammer-forged and chrome-lined, with an overall length of 263 mm (10.4 in). The barrel has eight rifling grooves with a right-hand twist rate of 1:231 mm (1:9.1 in), and it is equipped with a diagonally cut flash suppressor that also acts as a recoil compensator. The stated service life of the barrel is 20,000 rounds.
The P90 uses an internal hammer striking mechanism and a trigger mechanism with a three-position rotary dial fire control selector, located at the foot of the trigger. The dial has three settings: S – safe, 1 – semi-automatic fire, and A – fully automatic fire. When set on A, the P90's fire selector provides a two-stage trigger operation similar to that of the Steyr AUG assault rifle—pulling the trigger back slightly produces semi-automatic fire, and pulling the trigger fully to the rear produces fully automatic fire.