A semiconductor memory device includes a memory core and an input/output circuit. The memory core amplifies a signal of a memory cell to output the amplified signal through an input/output line pair in a read mode, receives a signal of the input/output line pair to store in the memory cell in a write mode, and electrically separates a bit line pair from the input/output line pair in response to a read column selection signal, a write column selection signal and a first data masking signal. The input/output circuit buffers and provided a signal of the input/output line pair to input/output pins, receives input data from the input/output pins, and buffers the received input data to provide the buffered input data to the input/output line pair. Thus, the semiconductor device can perform a fast data writing operation.