A positioning device for a mobile x-ray detector, an x-ray device containing such a mobile positioning device, and a method mutually align a mobile x-ray detector and an x-ray emitter in relation to one another. In order to enable exact mutual alignment or positioning of the mobile x-ray detector in relation to the x-ray emitter, a positioning device is used for a mobile x-ray detector. The positioning device contains at least one marker and/or at least one sensor for determining the position and/or the orientation of the positioning device and contains a number of connection elements for establishing a mechanical connection between the positioning device and the mobile x-ray detector.