Diarchy (or dyarchy) is a society or organization with two rulers on equal standing. Diarchies are known from ancient Sparta, Rome, Carthage as well as from Germanic (see Germanic king) and Dacian tribes.

Examples of modern forms are the governments of Andorra and San Marino.

Examples of English kings

• Hengest and Horsa
• Ethelred of Wessex and Alfred the Great

Examples of Swedish kings

• Erik and Alrik
• Yngvi and Alf
• Björn at Hauge and Anund Uppsale
• Eric the Victorious and Olof (II) Björnsson
• Eric the Victorious and Olof Skötkonung
• Haakon the Red and Ingold I