Hydraulic jack (1, 1′) comprising a main body (2), an articulated lifting arm (3) and a hydraulic cylinder (4) connected to the main body (2) by a first articulated connection (5) and to the lifting arm (3) by a second articulated connection (6), where the hydraulic jack (1, 1′) comprises a lever member (7, 7′) disposing of at least one locking zone (14) for receiving an element that is solidary to the lifting arm (3) and that serves as a stop, where the lever member (7, 7′) is articulately connected to the first articulated connection (5). The element solidary to the lifting arm (3) that serves as a stop and is capable of lodging in at least one locking zone (14) is comprised in the second articulated connection (6). This way, a mechanical locking system is provided which presents optimal performance while minimizing the number of necessary components.