A hydraulic jack is provided, with a main body and an articulated lifting arm, provided with safety mechanisms so that the jack load does not fall in the event of hydraulic failure. The jack includes a lever member that turns in relation to the main body, operated by a spring in a locking direction and operable in an unlocking direction by an operable area. The lever member includes a first curved area and a second curved area separated by a step. The lifting arm includes a rotating stop member capable of rolling along the first curved area or the second curved area when the lifting arm rotates, and which lodges itself in the step in order to lock the lifting arm and prevent it from rotating in a descending direction.