Year 780 (DCCLXXX) was a leap year starting on Saturday (link will display the full calendar) of the Julian calendar. The denomination 780 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.
|Ab urbe condita||1533|
|Balinese saka calendar||701–702|
|Chinese calendar||己未年 (Earth Goat)|
3476 or 3416
— to —
庚申年 (Metal Monkey)
3477 or 3417
|- Vikram Samvat||836–837|
|- Shaka Samvat||701–702|
|- Kali Yuga||3880–3881|
|Japanese calendar||Hōki 11|
|Minguo calendar||1132 before ROC|
|Seleucid era||1091/1092 AG|
|Thai solar calendar||1322–1323|
906 or 525 or −247
— to —
907 or 526 or −246
- September 8 - Emperor Leo IV ("the Khazar") dies after a 5-year reign, in which he has been dominated by his wife Irene of Athens, an iconodule. He is succeeded by his 9-year-old son Constantine VI; due to his minority, Irene and her chief minister Staurakios exercise a regency over him.
- Saxon Wars: King Charlemagne decrees the death penalty for any subdued Saxon refusing Christian baptism. Meanwhile Widukind builds up rebel strength in northern Saxony.
- The city of Osnabrück, developed as a marketplace, is founded by Charlemagne (approximate date).
- The city of Aldwych rises as an important trading centre in London, under Mercian control. King Offa of Mercia possibly establishes a royal palace at Aldermanbury, on the site of the old Roman Cripplegate fort and amphitheatre (approximate date).
- King Hyegong of Silla is killed in a revolt, terminating the kingly line of succession of former ruler Muyeol. He is the architect of Silla's unification of the Korean Peninsula (see 668).
- Ahmad ibn Hanbal, Muslim scholar and theologian (d. 855)
- Frederick of Utrecht, Frisian bishop (approximate date)
- Guifeng Zongmi, Chinese Buddhist monk (d. 841)
- Hugh of Tours, Frankish nobleman (approximate date)
- Muḥammad ibn Mūsā al-Ḵwārizmī, Persian mathematician (approximate date)
- Rabanus Maurus, archbishop of Mainz (approximate date)
- Yunyan Tansheng, Chinese Buddhist monk (d. 841)
- c. June 24 – Kume no Wakame, Japanese noblewoman
- August 19 – Credan, English abbot and saint
- September 8 – Leo IV, Byzantine emperor (b. 750)
- Dunchadh ua Daimhine, king of Uí Maine (Ireland)
- Hyegong, king of Silla (Korea) (b. 758)
- Khun Lo, founder of Luang Prabang (Laos)
- Approximate date – Himiltrude, queen consort of Charlemagne (b. c.742)