|Ab urbe condita||1542|
|Balinese saka calendar||710–711|
|Chinese calendar||戊辰年 (Earth Dragon)|
3485 or 3425
— to —
己巳年 (Earth Snake)
3486 or 3426
|- Vikram Samvat||845–846|
|- Shaka Samvat||710–711|
|- Kali Yuga||3889–3890|
|Japanese calendar||Enryaku 8|
|Minguo calendar||1123 before ROC|
|Seleucid era||1100/1101 AG|
|Thai solar calendar||1331–1332|
915 or 534 or −238
— to —
916 or 535 or −237
Year 789 (DCCLXXXIX) was a common year starting on Thursday (link will display the full calendar) of the Julian calendar. The denomination 789 for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.
- King Charlemagne crosses the Elbe River with a Frankish-Saxon army into Obotrite territory. He subdues the Wiltzes, and reaches the Baltic.
- King Pepin of Italy conquers Istria on the Adriatic, ignoring Byzantine protests. He establishes a tributary march, and sends missionaries.
- Charlemagne issues the Admonitio generalis, which covers educational and ecclesiastical reforms within the Frankish Kingdom.
- Charlemagne founds the town of Herford (modern Germany), in order to guard a ford crossing the narrow Werre River.
- King Beorhtric of Wessex marries Princess Eadburh, daughter of King Offa of Mercia, and accepts Mercian overlordship.
- Constantine I is installed as king of the Picts. He becomes one of the greatest Scottish monarchs in the Viking period.
- The Anglo-Saxon Chronicle records the first appearance of Vikings in England (see 787).
- Al-Khayzuran, widow of former Abbasid caliph Al-Mahdi, dies, leaving more of the effective power in the hands of Harun al-Rashid.
- Idris I reaches Volubilis and founds the Idrisid dynasty, ceding Morocco from the Abbasid caliphate and founding the first Moroccan state.
- An uprising in Japan leads to a major defeat for Emperor Kanmu, along with a severe drought and famine; the streets of the capital Nagaoka-kyō are clogged with the sick.
- February 20 – Leo of Catania, saint and bishop of Catania (b. 709)
- November 8 – Willehad, bishop of Bremen
- Al-Khayzuran, wife and adviser of Al-Mahdi, de facto co-ruler of the Abbasid Caliphate
- Fiachnae mac Áedo Róin, king of Ulaid (Ireland)
- Hildeprand, duke of Spoleto
- Mauregatus, king of Asturias (or 788)
- Torson, Frankish count of Toulouse (or 790)
- Jeep (2001), pp. 5–6