A semiconductor memory device and a method of expanding a valid output data window are described. The semiconductor memory device includes a memory cell array and an output circuit. The memory cell array generates read data having a plurality of bits. The output circuit outputs the, read data sequentially in response to a clock signal in a normal mode. On the other hand, the output circuit selectively outputs the bits of the read data by latching bits to be tested among bits of the read data, and by electrically disconnecting bits not to be tested among bits of the read data in response to a plurality of switch control signals in a test mode. Therefore a valid data window of an output data may be expanded.