The present technology relates to a data processing device and a data processing method capable of appropriately processing a stream. NPs included in a division stream of a predetermined channel acquired by dividing an input stream configured by a plurality of packets into division streams of a plurality of channels are eliminated, and a NP-eliminated stream including DNP representing the number of eliminated NPs is generated. Then, a stream including the NP-eliminated stream and signaling as a size identifier representing a size of the DNP or a stream including the NP-eliminated stream and a buffer capacity of a buffer storing the NP-eliminated stream and signaling as read start time from the buffer so as to generate a NP-inserted stream acquired by inserting NPs corresponding to a number represented by the DNP into the NP-eliminated stream is generated. The present technology, for example, can be applied to a channel bonding (CB) technology for dividing an input stream into a plurality of channels and transmitting divided streams.