A plate clamping device for clamping flexible printing plates in rotary printing machines includes an upper guiding rail and a lower guiding rail to receive and clamp a printing plate therebetween. The device is provided with two spaced from one another bolts and a clamping arrangement centrally positioned between said bolts. The clamping arrangement includes an eccentric stud extending normally to the elongation of the lower rail and a wedged key mounted between the upper rail and the stud. The stud has a surface having a flat portion, a transition curved portion and a cylindrical portion. The wedged key is arranged in engagement with this surface and upon rotation of the stud the key may rest on the flat portion of the stud when the device is in a plate-releasing position or the key may engage with the cylindrical portion of the stud whereby the key is self-arrested and the rails clamp a printing plate.