A semiconductor memory device includes a memory cell array, a controller, and a data input/output (I/O) unit. The memory cell array includes a plurality of memory cells and is configured to store data. The controller is configured to enable a write clock signal in response to an active command when a write latency of the semiconductor device is less than a reference write latency and disable the write clock signal during a disabling period in which read data is output from the semiconductor device. The data I/O unit is configured to receive data in response to the write clock signal and output the data to the memory cell array.