The invention relates to a length-adjustable con rod (1) for a reciprocating piston engine, particularly an internal combustion engine, having at least a first and a second rod part (2, 4), wherein the two rod parts (2, 4) can be pushed together and/or inside one another in a telescoping manner, wherein the second rod part (4) forms a guide cylinder (4a) and the first rod part (2) a piston element (3) which can be pushed longitudinally in the guide cylinder (4a), wherein a high-pressure chamber (4b), into which at least one oil channel (10) opens, is clamped between the first and the second rod parts (2, 4). To enable the compression ratio to be adjusted in the simplest and most reliable manner possible, according to the invention the two rod parts (2, 4) can be fixed, by a locking device (32), relative to one another in at least one position, preferably corresponding to a minimum length of the con rod (1).