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