Poseidonius or Posidonius may refer to:

• Posidonius Stoic philosopher, politician, astronomer, geographer, historian

• The Madonna of the Mountains
• The Madonna of the Pinks
• The Madonna's Secret
• The Mad Peck
• The Mad Phoenix
• The Mad Pooper
• The Mad Queen
• The Madrasa of Abulkosim
• The madrasa of Abulkosim
• The Madras Courier
• The Madras Standard
• The Madrigal Society
• The Mad Room
• The Mad Russian (disambiguation)
• The Mad Russian (wrestler)
• The Mad Titan
• The Mad Train
• The Madurai Diraviyam Thayumanavar Hindu College
• The Mad Whale
• The Madwoman in the Volvo
• The Maelstrom
• The Maelstrom of Paris
• The Maestro (album)
• The Maestro: King of the Cowboy Artists
• The mafia