A memory device is configured to perform a parallel read-modify-write operation, which generates a syndrome for first partial data read from a memory cell array and second partial data according to a data mask option, corrects an error of the second partial data, generates merged data by combining second data with the corrected second partial data, and generates an internal parity for the merged data. The memory device sets the second partial data to binary values of 0, generates third data by combining the second partial data set to the binary values of 0 with the second data, generates a third parity for the third data, and generates an internal parity for the merged data based on the syndrome, a partial parity, and the third parity.