List of state leaders in 840

This is a list of heads of state, government leaders, and other rulers in the year 840.

Africa

• Abbasid Caliphate – Al-Mu'tasim (833–842)

Asia

• Abbasid Caliphate – Al-Mu'tasim (833–842)
• Byzantine Empire – Theophilos (829–842)
• China (Tang dynasty) –
  1. Emperor Wenzong (826–840)
  2. Emperor Wuzong (840–846)
• Pratihara Empire – Mihira Bhoja I (c. 836–886)
• Japan (Heian period) – Emperor Ninmyō (833–850)
• Korea (North South States Period)
  • Balhae – Dae Ijin (830–857)
  • Unified Silla – Munseong (839–857)

Europe

• First Bulgarian Empire – Presian, Khan of the Bulgars (836–852)
• Byzantine Empire – Theophilos (829–842)
• Caucasus
  • Principality of Iberia – Bagrat I Kuropalates (830–876)
  • Principality of Kakheti – Samuel (839–861)
• Kingdom of Denmark – Horik I (813–854)
• England (Heptarchy) –
  • Kingdom of East Anglia – Æthelweard (839–854)
  • Kingdom of Kent – Æthelwulf of Wessex (839–856)
  • Kingdom of Mercia –
    1. Wigmund (839–840)
    2. Wigstan (840)
    3. Beorhtwulf (840–852)
  • Kingdom of Northumbria – Eanred (810–841)
  • Kingdom of Wessex – Æthelwulf (839–856)
• Frankish Empire –
  • Louis the Pious, King of the Franks and senior Holy Roman Emperor (814–840)
  • Lothair I, King of Italy and junior Holy Roman Emperor (817–855)
  • County of Angoulême – Turpio (839–863)
  • Kingdom of Aquitaine –
  • Pepin II of Aquitaine (838–851/852)
  • Charles the Bald (838–855)
    • Auvergne – Gerard, Count of Auvergne (839–841)
    • County of Vasconia – Sans II Sancion of Gascony (836–864)
    • County of Toulouse – Bernard of Septimania (835–842)
  • County of Barcelona – Bernard of Septimania (835–844)
  • Duchy of Bavaria – Louis the German (817–843)
  • Duchy of Croatia – Mislav of Croatia (835–845)
  • County of Maine –
    1. Rorgon I (832–840)
    2. Gauzbert (840–853)
  • County of Nantes – Ricwin (831–841)
  • County of Poitiers –
    1. Renaud I (839–840)
    2. Bernard II (840–844)
  • County of Urgell – Sunifred, Count of Barcelona (834–848)
• Ireland – Niall Caille, High King of Ireland (833–846)
  • Kingdom of Ailech – Niall Caille (823–846)
  • Kingdom of Connacht
    1. Murchad mac Áedo (839–840)
    2. Fergus mac Fothaid (840–843)
  • Uí Maine – Cathal mac Ailell (834–844)
  • Kingdom of Leinster – Ruarc mac Brain (838–847)
  • Kingdom of Mide – Máel Ruanaid mac Donnchada Midi (833–843)
  • Kingdom of Munster – Fedelmid mac Crimthainn (820–847)
  • Kingdom of Ulaid – Matudán mac Muiredaig (839–857)
• Kingdom of Italy – Lothair I, King of Italy and junior Holy Roman Emperor (818–855)
  • Principality of Benevento – Radelchis I (839–851)
  • Duchy of Gaeta – Constantine, Hypatus of Gaeta (839–866)
  • Duchy of Naples – Sergius I (840–864)
  • Papal States – Pope Gregory IV (827–844)
  • Principality of Salerno – Siconulf (839–851)
  • Duchy of Spoleto – Berengar (836–841)
  • Republic of Venice – Pietro Tradonico, Doge of Venice (836–864)
• Great Moravia – Mojmir I (831–846)
• Scotland
  • Kingdom of the Picts – Uurad (839–842)
• Principality of Serbia – Vlastimir (835–850)
• Spain –
  • County of Aragon – Galindo Garcés (833–844)
  • Kingdom of Asturias – Alfonso II (791–842)
  • Emirate of Córdoba – Abd ar-Rahman II (822–852)
  • Kingdom of Pamplona – Íñigo Arista (824–851/852)
• Wales –
  • Kingdom of Gwynedd – Merfyn Frych (825–844)
  • Kingdom of Powys – Cyngen ap Cadell (808–855)