A thermoforming tool for producing cup-shaped molded parts from a heated film of plastic includes an upper tool and a lower tool, which are movable in relation to each other for clamping the film of plastic therebetween. At least one mold cavity for forming a molded part from the film of plastic is formed in the lower tool and at least one prestretching structure movable into a corresponding mold cavity is arranged in the upper tool inside a hold-down device. The prestretching structure includes at least two prestretching levers pivotable about an associated pivot axis, which lies in a plane running perpendicularly to the direction of movement of the prestretching structure. Each of the prestretching levers has a stretching arm extending downward, and a stop arm extending upward. On the inner surface of the hold-down device a guiding projection is formed opposite to a corresponding one of the prestretching levers, the stretching arm sliding on the guiding projection during movement of the prestretching structure into the mold cavity in such a way that the stretching arm is pivoted outward.