An improved backpack sprayer, comprising a supply tank, an expandable accumulator, a pump mechanism, an intake valve, a discharge valve, and a spray wand with a nozzle. The supply tank is designed to hold fluids, such as insecticides, herbicides, water sealants, etc. The expandable accumulator is preferably an elastomeric bladder, the function of which is to accumulate fluid from the supply tank under pressure. The volume of the bladder expands as pressurized fluid enters from the pump mechanism. As with most backpack sprayers, the pump mechanism is positioned adjacent to the bottom of the supply tank and is in fluid communication with the tank. The function of the pump mechanism is to pump fluid from the supply tank into the bladder. An intake valve, associated with an inlet opening of the bladder, enables a unidirectional flow of liquid from the pump to the bladder. Discharge of fluid from the bladder also occurs through the inlet opening upon the opening of a discharge valve. The discharge valve is normally closed to permit the bladder to accumulate a desired quantity of fluid. In the preferred embodiment, the discharge valve is hand-operated and located in a handle attached to a spray wand and nozzle. When the discharge valve is opened, the discharged fluid is directed from the accumulator and channeled through a hose, trigger handle, spray wand and spray nozzle.