The invention relates to a locking device (5) for locking together a first tube (2) and a second tube (3), the first tube being slidably received in the second tube, the locking device comprising at least one rod (10) mounted to move into two positions and a cam system made up of at least two cams (6, 7, 8, 9) that are mounted on the first tube to pivot either into a first position in which the cams are not in contact with the second tube and allow the first tube to slide inside the second tube, or into a second position in which each of the cams exerts pressure on the inside face of the second tube and prevents the first tube from sliding inside the second tube. The rod is assembled with the cam system and is configured so that it at least moves the cams into their first position, a repositioning system being configured to return said cams into their second position. The invention also relates to a bicycle (1) equipped with such a locking device (5) arranged between a saddle post (2) and a saddle tube (3).