A semiconductor memory device includes a memory device, a Read-Modify-Write (RMW) controller configured to generate a merge command corresponding to at least one command among a read command and a write command which are externally provided, to receive a processing result of the merge command, and to generate a response for the at least one command corresponding to the merge command. The semiconductor memory device further includes a memory controller configured to control the memory device by receiving the merge command and to provide the processing result of the merge command to the RMW controller.