A rotary valve actuator for actuating a valve that must be rotated between an open position and a closed position. The valve is disposed in a tubular valve housing having an exterior surface with at least one actuating member receiving aperture. The rotary valve actuator includes a substantially annular body having an interior surface, an exterior surface and a central axis. A male actuator member is rotatably mounted within the body. The male actuator member has a valve coupling end that projects perpendicularly from the interior surface of the body, such that it can be inserted into the actuating member receiving aperture of the tubular valve housing. A hydraulic chamber is positioned in the body having a first inlet and a second inlet. A fluid responsive member is positioned in the one hydraulic chamber between the first inlet and the second inlet. Hydraulic fluid is selectively supplied to the first inlet, such that the flow of hydraulic fluid through the first inlet into the hydraulic chamber causes the fluid responsive member to move in a first direction. Hydraulic fluid is selectively supplied to the second inlet, such that the flow of hydraulic fluid through the second inlet into the hydraulic chamber causes the fluid responsive member to move in a second direction. The movement of the fluid responsive member is translated into rotary movement of the male actuator member.