The invention relates to a device (1) for adjusting the camshaft of an internal combustion engine, which has an internal rotor (2) which can be adjusted rotationally in relation to an external rotor (3), with the internal rotor (2) being secured to a camshaft (5) by means of a central screw (4). In order to permit improved mounting of the device it is provided according to the invention for the internal rotor (2) and/or the external rotor (3) to have at least one recess (6, 7) for the engagement of a mounting tool (8, 9) by means of which the internal rotor (2) and the external rotor (3) can be secured fixed in terms of rotation. Furthermore, the invention relates to a mounting tool for mounting such a device.