A pneumatic speargun for spearfishing and a related method of loading and firing includes a cylinder having a head and a body with a hand grip, a slide guide for a spear, a reservoir for a pressurized fluid, and a barrel connected to the reservoir. A piston is slidingly mounted in the barrel for movement from an end loading positions, in which it is retained, to an end discharge position. A trigger releases the retaining system and causes the piston to move from the selected end loading position to the end discharge position. The piston in the loading position is removably connected to the spear via a cable and a return system and the spear becomes propelled as the piston moves from the end discharge position to the selected end loading position by pulling the cable.