A pretensioning tool is provided for an eccentric tensioning device, which is used for tensioning a power-transmission element constructed, for example, as a flat or toothed belt in a power-transmission drive. The tensioning tool has an engagement structure that can be coupled so that it is locked in rotation with a working eccentric from a front region facing away from the flange side of the eccentric tensioning device and a decoupling structure for implementing an axial displacement of the working eccentric relative to the bearing journal. In this way, it is possible to turn back the working eccentric by hand and with moderate hand forces against the tensioning moment applied by the torsion spring of the tensioning device into a mounting position and to secure the working eccentric in this mounting position through axial decoupling of this eccentric relative to a base plate device.