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.
|684 by topic|
|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
- 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 – 684 Hakuho earthquake. A great earthquake strikes Japan. The people, houses, temples, shrines and domestic animals are greatly damaged.
- 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
- Severus II bar Masqeh, Syriac Orthodox Patriarch of Antioch.