Year 684 (DCLXXXIV) was a leap year starting on Friday (link will display the full calendar) of the Julian calendar. The denomination 684 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||1437|
|Balinese saka calendar||605–606|
|Chinese calendar||癸未年 (Water Goat)|
3380 or 3320
— to —
甲申年 (Wood Monkey)
3381 or 3321
|- Vikram Samvat||740–741|
|- Shaka Samvat||605–606|
|- Kali Yuga||3784–3785|
|Minguo calendar||1228 before ROC|
|Seleucid era||995/996 AG|
|Thai solar calendar||1226–1227|
810 or 429 or −343
— to —
811 or 430 or −342
- Ghislemar, mayor of the palace in Neustria and Burgundy, dies after a 2-year reign, and is succeeded by his father Waratton. He makes peace between the three Frankish kingdoms.
- King Ecgfrith of Northumbria sends a punitive expedition to Ireland under his ealdorman Berht, laying waste to the territory of Meath, ruled by High King Fínsnechta Fledach.
- Caliph Muawiya II dies at Damascus, after a brief reign that ends Sufyanid rule. A new caliph is proclaimed in Syria amidst tribal wars, but Marwan I will reign until next year.
- August 18 – Battle of Marj Rahit: Muslim partisans under Marwan I defeat the supporters of Abd Allah ibn al-Zubayr near Damascus, and cement Umayyad control of Syria.
- January 3 – Zhong Zong succeeds his father Gao Zong, and becomes emperor of the Tang Dynasty. His mother Wu Zetian remains the power behind the throne in China.
- February 27 – Wu Zetian replaces Zhong Zong in favor of his younger brother Rui Zong. He becomes a puppet ruler, and Zhong Zong is placed under house arrest.
- Summer – The Pallava Empire (modern India) invades the kingdom of Ceylon. A Pallavan naval expedition employing Tamil mercenaries ends the Moriya Dynasty.
- September 7 – A large comet is observed in Japan (it's Japan's oldest observation record of the Halley's Comet).
- November 13 – Emperor Tenmu institutes eight titles of eight classes (Yakusa-no-kabane) in Japan.
- November 26 – A great earthquake strikes Japan. The people, houses, temples, shrines and domestic animals are greatly damaged.
- February 10 – K'inich Kan B'alam II accedes to the rulership of the Maya polity of Palenque (modern Mexico).
- Cuthbert is elected Bishop of Hexham, and receives a visit from a large group under Ecgfrith. He agrees to return to Lindisfarne (Northumbria) to take up duties.
- June 26 – Pope Benedict II succeeds Leo II as the 81st pope of Rome, after a period of sede vacante ("vacant seat") of 1 year.
- Adarnase II, king of Iberia (approximate date)
- Aldegonde, Frankish Benedictine abbess
- Constantine of Mananali, founder of the Paulicians
- Ghislemar, mayor of the palace in Neustria and Burgundy
- Li Xian, prince of the Tang Dynasty (b. 653)
- Luo Binwang, Chinese poet and official
- Muawiya II, Muslim caliph (b. 661)
- Pei Yan, chancellor of the Tang Dynasty
- Philibert of Jumièges, Frankish abbot