The mystikos (Greek: μυστικός, "the secret one") was an important Byzantine office of the imperial chancery from the 9th through to the 15th centuries. Its initial role is unclear; he was probably the Byzantine emperor's private secretary. In time, the office also exercised judicial duties. It became an important fiscal official in the Komnenian period, and remained one of the highest-ranking state offices into the Palaiologan period as well.