|Centuries:||7th century – 8th century – 9th century|
|Decades:||750s 760s 770s – 780s – 790s 800s 810s|
|Years:||781 782 783 – 784 – 785 786 787|
|784 by topic|
|State leaders – Sovereign states|
|Birth and death categories|
|Births – Deaths|
|Establishment and disestablishment categories|
|Establishments – Disestablishments|
|Ab urbe condita||1537|
|Chinese calendar||癸亥年 (Water Pig)
3480 or 3420
— to —
甲子年 (Wood Rat)
3481 or 3421
|- Vikram Samvat||840–841|
|- Shaka Samvat||706–707|
|- Kali Yuga||3885–3886|
|Japanese calendar||Enryaku 3
|Minguo calendar||1128 before ROC
|Seleucid era||1095/1096 AG|
|Thai solar calendar||1326–1327|
Year 784 (DCCLXXXIV) was a leap year starting on Thursday (link will display the full calendar) of the Julian calendar. The denomination 784 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.
- Saxon Wars: King Charlemagne begins a campaign in northern Saxony. He ravages Eastphalian territory as far as the Elbe River while his son, Charles the Younger, defeats a Saxon force in the Lippe Valley. Bad weather hinders Charlemagne's winter campaign in southern Saxony.
- Winter – Charlemagne returns to Eresburg and builds a church, probably on the site of the Irminsul (a pagan religious site). Frankish forces based at Eresburg attack 'rebel' Saxon settlements and take control of the roads. Charlemagne himself takes part in some of these raids.
- Abd al-Rahman I, emir of Córdoba (Al-Andalus), begins with the construction of the Prayer Hall, the Great Mosque of Córdoba. He uses the mosque (originally called Aljama Mosque) as an adjunct to his palace and names it in honour of his wife.
- The Japanese begin a war against the Ainu – in the north on the main island of Honshu. Emperor Kanmu wishes to be free of influence from the Buddhist monasteries around Nara (then called Heijō), and moves the capital to Nagaoka, ending the Nara period.
- August 30 – Paul IV abdicates as patriarch of Constantinople.
- December 25 – Tarasios is elected patriarch of Constantinople.
- Ibn Sa'd al-Baghdadi, Muslim historian (d. 845)
- Theodrada, daughter of Charlemagne (approximate date)
- Alberic, bishop of Utrecht
- May 4 – Arbeo, bishop of Freising
- Autpert Ambrose, Frankish abbot
- July 16 – Fulrad, Frankish abbot (b. 710)
- Isa ibn Musa, Muslim governor (or 783)
- Paul IV, patriarch of Constantinople
- Vergilius, bishop of Salzburg