Abstract: The present technology relates to a demodulation device, a processing device, a reception device, and a data processing method for more flexibly coping with change in transmission method. Provided is a demodulation device including a demodulation unit configured to demodulate a first transmission packet obtained from a broadcast signal, and an output unit configured to output a divided packet via a predetermined interface, the divided packet being obtained by dividing the first transmission packet that is a variable-length packet used in a first transmission method into a packet length according to a second transmission packet that is a fixed-length packet used in a second transmission method and arranging the first transmission packet in a payload, and adding a header including information for restoring the first transmission packet to the payload. The present technology can be applied to, for example, a demodulation IC incorporated in a television receiver or a set top box.