840年
840年 — 中国: Uyghur Khaganate enters decline in the aftermath of civil war / 日本: Nihon Kōki (3rd volume of historical chronicles Rikkokushi) was completed
事件5条。正文照录原始年表原文，来源见来源与许可。上下灰色行是前后2年的脉络。
| 韩国南北国时代 698–918 | 中国唐朝 618–907 | 日本平安时代 794–1185 | 美国 |
|---|
- 838신라 민애왕 즉위
- 839신라 문성왕 즉위
- 839신라 장보고의 도움으로 신무왕 즉위
| - Uyghur Khaganate enters decline in the aftermath of civil warEN
- February — Emperor Wenzong of Tang dies; his brother Li Chan is placed on the throne by eunuchs and becomes Emperor Wuzong of TangEN
- February — Wenzong's brother Emperor Wuzong of Tang became emperor of the Tang dynasty.EN
- February — Wenzong died.EN
- 838Japanese monk Ennin visits China
- 841唐武宗即位，是為會昌中興。
- 842Tibetan Empire enters decline after the death of Dharma
| - Nihon Kōki (3rd volume of historical chronicles Rikkokushi) was completed.EN
- 839Last envoy to Tang China sent (some later embassies were cancelled).
- 842承和之變
- 842Emperor Montoku ascends to the throne.
| 1607年起收录 |