An encryption/decryption apparatus and method using an advanced encryption standard (AES) Rijndael algorithm are provided. The apparatus includes a round key operator that performs arithmetic operations on a round key for a first round and first partial round keys of round keys for second to last rounds and generates the round keys for the second to last rounds, and a round executor that performs an encryption or decryption operation using the round key for the first round and the round keys for the second to last rounds.