A controller for a vehicle power transmission system includes: an electric differential unit that includes a differential mechanism, having an input shaft and an output shaft, and an electric motor and that allows controlling a differential state between the rotational speed of the input shaft coupled to an engine and the rotational speed of the output shaft by controlling an operating state of the electric motor coupled to a rotating element of the differential mechanism; a transmission unit that is arranged in a power transmission path between the electric differential unit and a drive wheel; and an engine upper limit setting device that, when the transmission unit is shifting speeds, prelimits an upper limit of the rotational speed of the engine when the rotational speed of the engine increases because of torque reduction control by the electric motor. With the above control, the rotational speed of the engine is prevented from reaching a high rotational speed region.