List of state leaders in 821

Asia

• Bohai/Balhae - Da Renxiu/Dae In-su, King of Bohai/Balhae (818?-830)
• China (Tang Dynasty) - Muzong , Emperor of Tang China (821-824)
• Gurjara - Nagabhata II , Pratihara King of Gurjara (805-833)
• Japan (Heian Period) - Saga, Emperor of Japan (809-823)
• Silla - Heondeok , King of Silla (809-826)

Europe

• Dalriada - Oengus, King of Dalriada (820–834)
• East Anglia -
  1. Coenwulf , King of East Anglia (799–821)
  2. Coelwulf I, King of East Anglia (821–823)
• Essex - Sigered , King of Essex (798–825)
• Kent - Baldred , King of Kent (807–825)
• Mercia -
  1. Coenwulf, King of Mercia (796–821)
  2. Coenelm , King of Mercia (821)
  3. Ceolwulf I, King of Mercia (821–823)
• Northumbria - Eanred, King of Northumbria (810–841)
• Ireland - Conchobar, High King of Ireland (818–831)
• Picts - Oengus II , King of the Picts (820–834)
• Strathclyde - Arthgal f Dyfnwal , King of Strathclyde (816–872)
• Wales - Hywel Farf-Fehinog ab Caradog , King of Gwynedd (814–825)
• Wessex - Egbert of Wessex, King of Wessex and Bretwalda (802–839)