A semiconductor memory device includes an error correction code (ECC) engine, a memory cell array, an input/output (I/O) gating circuit and a control logic circuit. The memory cell array includes a normal cell region configured to store main data and a parity cell region configured to selectively store parity data which the ECC engine generates based on the main data, and sub data received from outside of the semiconductor memory device. The control logic circuit controls the ECC engine to selectively perform an ECC encoding and an ECC decoding on the main data and controls the I/O gating circuit to store the sub data in at least a portion of the parity cell region.