Didymus may refer to:
- Thomas the Apostle or "Didymus", both names meaning "twin" in Aramaic and Greek respectively, is the most well-known Didymus due to his role in early Christian history
- Didymus Chalcenterus (ca. 63 BCE – 10 CE), Hellenistic scholar and grammarian
- Didymus the Blind (313–398), ecclesiastical writer of Alexandria
- Didymus the Musician, music theorist in Alexandria of the 1st century AD
- Theodora and Didymus, 4th century Christian saints
- Didymus Mutasa, Minister of National Security in Zimbabwe
- Leptodactylus didymus, a species of frog in Bolivia and Peru
- A member of the House of Theodosius and distant relative of the Roman emperor Flavius Augustus Honorius, of whom little is known.
- Sir Didymus, a fictional character from the movie Labyrinth.