A gearbox assembly for a twin-screw extruder comprises a speed change gearbox for adjusting a screw shaft speed of two screw shafts of the twin-screw extruder, a distribution gearbox allowing the screw shafts to be coupled thereto, and a speed reduction gearbox. The speed change gearbox and the speed reduction gearbox are configured such as to be separable from each other, wherein the speed change gearbox is arranged at a driving end while the speed reduction gearbox couples the speed change gearbox to the distribution gearbox. Due to the fact that the speed change gearbox is configured as an individual gearbox unit arranged at the driving end, an adjustment of the screw shaft speed is easily possible by replacing the speed change gearbox. As a result, a high flexibility and productivity are obtained in the operation of the twin-screw extruder.