Adjustable-length connecting rod (100, 200) for a reciprocating piston engine, in particular for a reciprocating piston internal combustion engine, having a hydraulic cylinder for adjusting an effective connecting rod length (L) of the connecting rod (100, 200), which hydraulic cylinder has at least one piston and a first hydraulic operating chamber (21) and a second hydraulic operating chamber (22), a hydraulically actuable control device (8) which can be switched over at least between two switching states with an actuating piston (23) for controlling the adjustment of the connecting rod (100, 200), and a mechanically actuable actuating device (9) for switching the control device (8), which actuating device (9) is operatively connected via at least one hydraulic actuating line (11, 12) to the actuating piston (23) of the control device (8) for actuating it hydraulically.