Disclosed are a semiconductor memory device, a controller, and a memory system. The semiconductor memory device includes a memory cell array including a plurality of memory cells, and an error correcting code (ECC) decoder configured to receive first data and a parity output from selected memory cells of the memory cell array. The ECC decoder generates a syndrome based on the first data and the parity, generates a decoding status flag (DSF) indicating a type of an error of the first data by the syndrome, and outputs the second data and the DSF to an external device outside of the semiconductor memory device when a read operation of the semiconductor memory device is performed.