Dionysius I

Dionysius I may refer to:


 * Dionysius I of Syracuse (ca. 432–367 BC), Greek tyrant of Syracuse
 * Dionysius I (Syriac Patriarch), or Dionysius Telmaharensis (died 845), Patriarch of the Syriac Orthodox Church
 * Dionysius I, Metropolitan of Moscow (died 1385)
 * Patriarch Dionysius I of Constantinople (died 1492), Patriarch of Constantinople
 * Mar Dionysius I (died 1808), Metropolitan of the Malankara Church