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) - Emperor Saga, Emperor of Japan (809-823)
*Silla - Heondeok, King of Silla (809-826)

Europe

*Asturias - Alfonso II, King of Asturias (791-842)
*Bulgaria - Omurtag, Khan of Bulgaria (814–831)
*Dál Riata - Óengus, King of Dalriada (820–834)
*East Anglia -
*#Coenwulf, King of East Anglia (799–821)
*#Coelwulf I, King of East Anglia (821–823)
*Essex - Sigered, King of Essex (798–825)
*Frankish Empire - Louis the Pious, Holy Roman Emperor (816-840)
*Mercia -
*#Coenwulf, King of Mercia (796–821) & King of Kent (807–821)
*#Coenelm, King of Mercia (821) & King of Kent (821)
*#Ceolwulf I, King of Mercia (821–823) & King of Kent (821–823)
*Northumbria - Eanred, King of Northumbria (810–841)
*Ireland - Conchobar mac Donnchada, High King of Ireland (818–831)
*Picts - Óengus (II), King of the Picts (820–834)
*Strathclyde - Arthgal f Dyfnwal, King of Strathclyde (816–872)
*Venice - Agnello Participazio, Doge of Venice (811-827)
*Wales - Hywel "Farf-Fehinog" ab Caradog, King of Gwynedd (814–825)
*Wessex - Egbert of Wessex, King of Wessex and Bretwalda (802–839)