A mold clamping device for an injection-molding machine includes a fixed platen, a movable platen, pressure pads for generating a clamping force, and a spindle drive for moving the movable platen. The spindle drive has a non-rotating spindle shaft and a spindle nut which can be rotationally driven by a motor. The spindle shaft has a threaded portion in engagement with a matching thread of the spindle nut. To permit a free relative movement between the spindle drive and the movable platen, a piston-cylinder unit is in operative connection with the spindle drive and has on both sides of the piston pressure medium spaces, which can optionally be hydraulically blocked or connected to one another. In the latter state, the spindle drive is decoupled from the movable platen and freely floating with respect to it.