An underwater firearm is a firearm designed for use underwater. Underwater firearms or needleguns usually fire flechettes or spear-like bolts instead of standard bullets. These may be fired by pressurised gas.

A needlegun, also known as a needler, flechette gun or fletcher, is a firearm that fires small, sometimes fin-stabilized, metal darts or flechettes. Theoretically, the advantages of a needlegun over conventional projectile firearms are in its compact size, high rate of fire, and extreme muzzle velocity. The needle presents less frontal area than a bullet, producing less drag and thus more effective range than a wider projectile of the same mass and velocity. There have been experiments to make guided flechettes that can home in on targets.