Year 800 (DCCC) was a leap year starting on Wednesday (link will display the full calendar) of the Julian calendar. It was around this time that the Anno Domini calendar era became the prevalent method in Europe for naming years, so from this time on, the years began to be known as 800 and onwards.
|Ab urbe condita||1553|
|Balinese saka calendar||721–722|
|Chinese calendar||己卯年 (Earth Rabbit)|
3496 or 3436
— to —
庚辰年 (Metal Dragon)
3497 or 3437
|- Vikram Samvat||856–857|
|- Shaka Samvat||721–722|
|- Kali Yuga||3900–3901|
|Japanese calendar||Enryaku 19|
|Minguo calendar||1112 before ROC|
|Seleucid era||1111/1112 AG|
|Thai solar calendar||1342–1343|
926 or 545 or −227
— to —
927 or 546 or −226
- December 25 – Charlemagne, king of the Franks, is crowned Holy Roman Emperor by Pope Leo III as Charles I, with the title "Emperor of the Franks and the Lombards". The coronation takes place during Mass at the Basilica of St. Peter in Rome, on Christmas Day. The Frankish Empire is formed in Western Europe, which is not recognized by Empress Irene at Constantinople. This triggers a series of disputes with the Byzantines around who is officially ruling the former Western Roman Empire. 
- The Rus' Khaganate (modern Ukraine) is created by people who are called Rus', after the 182-year dominance of the Khazars. This is the starting period of the rise of the Kievan Rus', and the later states of Belarus and Ukraine (approximate date).
- King Eardwulf of Northumbria has his men seize prince Ealhmund, son of the late king Alhred, and put him to death. He is buried at Derby in St. Alkmund's Church, and later revered as a saint (approximate date).
- The ci, a new type of lyric poetry with irregular lines, is set to a melody during the Tang Dynasty (approximate date).
- Sijilmasa (in present-day Morocco) is founded as the departure point, for caravans between North Africa and the western Sudan (approximate date).
- Ife, in present-day Nigeria, becomes an important urban center (approximate date).
- The first settlers of the Hawaiian Islands arrive (approximate date).
- Archbishop Hygeberht of Lichfield retires; he is succeeded by Ealdwulf. King Coenwulf of Mercia is on better terms with the archepiscopal of Canterbury than his predecessor, and unsuccessfully attempts to have the Mercian archepiscopal see transferred to London (approximate date).
- The Book of Kells is written and illuminated in a Columban monastery, in the British Isles (approximate date).
- Aldric, bishop of Le Mans (approximate date)
- Al-Abbās ibn Said al-Jawharī, Muslim mathematician
- Álvaro of Córdoba, Mozarab scholar (approximate date)
- Amoghavarsha I, king of Rashtrakuta (India) (d. 878)
- Boso the Elder, Frankish nobleman (approximate date)
- Fatima al-Fihri, Arab university founder (approximate date)
- Govindasvāmi, Indian astronomer (approximate date)
- Louis, Frankish abbot (approximate date)
- Methodios I, patriarch of Constantinople (or 788)
- Nicholas I, pope of the Catholic Church (d. 867)
- Nominoe, duke of Brittany (approximate date)
- Pribina, Slavic prince (approximate date)
- Robert III, Frankish nobleman (d. 834)
- Rorgon II, count of Maine (approximate date)
- Swithin, bishop of Winchester (approximate date)
- Valentine, pope of the Catholic Church (d. 827)
- June 3 – Staurakios, Byzantine chief minister
- September 26 – Berowulf, bishop of Würzburg
- Ailill mac Fergusa, king of South Brega (Ireland)
- Alkelda, Anglo-Saxon princess (approximate date)
- Beatus of Liébana, monk and theologian (approximate date)
- Ealhmund, prince of Northumbria (approximate date)
- Luitgard, Frankish queen and wife of Charlemagne
- Vatsraja, king of the Gurjara-Pratihara Dynasty