A gas cylinder actuator with overtravel safety device, comprising a tubular jacket for gas containment, which is closed hermetically at one end by a bottom provided with a gas filling valve and at the opposite end by a head portion, which is provided with a hole for the passage of a stem with a piston, the jacket, the bottom and the piston forming the gas expansion and compression chamber.The head portion comprises an annular body, which is fixed internally to the jacket, and is provided with a central hole for the passage of the stem with the interposition of dynamic sealing elements, static sealing elements being interposed between the annular body and the jacket and an element for controlling the descending motion of a slider of a press with which the actuator is associated being provided and protruding from the annular body or from the jacket, the control element being preset to selectively move or break or deform in order to break or deform or render ineffective in general the static and dynamic sealing elements.