Melanthius (Greek: Μελάνθιος) may refer to:

• Melanthius, a painter of ancient Greece
• Melanthius (tragic poet), a 5th-century BC Athenian tragic poet known only as a satirical target for Aristophanes in the plays Peace and The Birds
• Melanthius, an ancient Athenian historian of the 3rd or 2nd century BC; he wrote a history of Attica and a book on the Eleusinian Mysteries, of which only fragments remain.
• Melanthius (Odyssey), a minor character in Homer's Odyssey
• Melanthius, a character in the fantasy film Sinbad and the Eye of the Tiger
• Melanthius (crater), an impact crater on Tethys, a moon of Saturn
• Melanthius, a quadrangle of the moon Tethys