|781 by topic|
|Ab urbe condita||1534|
|Balinese saka calendar||702–703|
|Chinese calendar||庚申年 (Metal Monkey)|
3477 or 3417
— to —
辛酉年 (Metal Rooster)
3478 or 3418
|- Vikram Samvat||837–838|
|- Shaka Samvat||702–703|
|- Kali Yuga||3881–3882|
|Japanese calendar||Hōki 12 / Ten'ō 1|
|Minguo calendar||1131 before ROC|
|Seleucid era||1092/1093 AG|
|Thai solar calendar||1323–1324|
907 or 526 or −246
— to —
908 or 527 or −245
Year 781 (DCCLXXXI) was a common year starting on Monday (link will display the full calendar) of the Julian calendar. The denomination 781 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 has his son Carloman (renamed Pepin) anointed "King of Italy", and he is crowned by Pope Adrian I with the Iron Crown of Lombardy. His younger brother Charles I is anointed king of Aquitaine, and Louis the Pious (only 3-years old) is appointed sub-king of Italy and Aquitaine.
- Charlemagne meets Alcuin, Anglo-Saxon missionary, in Italy, and invites him to Aachen, where he becomes Charlemagne's chief adviser on religious and educational matters (approximate date).
- Yang Yan, Chinese statesman, commits suicide after being accused of bribery and corruption. He is credited with reforming the tax system for peasants, reducing the power of the aristocratic classes, and eliminating their tax-free estates.
- April 30 – Emperor Kōnin of Japan abdicates the throne after an 11-year reign, in favour of his half-Korean son, Kanmu.
- July 31 – The oldest recorded eruption of Mount Fuji occurs (Traditional Japanese date: July 6, 781).
- New city of Bian (汴) is constructed on the site of Kaifeng during the Tang Dynasty (China).
- Charlemagne defines the Papal territory (see Papal States). He codifies the regions over which the pope would be temporal sovereign: the Duchy of Rome is expanded by Ravenna, the Duchy of the Pentapolis, parts of the Duchy of Benevento, Tuscany, Corsica and Lombardy.
- Nestorians in China build Christian monasteries, and erect the Nestorian Stele (approximate date).
- Harith al-Muhasibi, founder of the Baghdad School of Islamic philosophy, and a teacher of the Sufi masters Junayd al-Baghdadi and Sari al-Saqti. (d. 857)
- Alchmund, bishop of Hexham (approximate date)
- Fergus mac Echdach, king of Dál Riata (Scotland)
- Guo Ziyi, general of the Tang Dynasty (b. 697)
- Isonokami no Yakatsugu, Japanese nobleman (b. 723)
- Yang Yan, chancellor of the Tang Dynasty (b. 727)