Year 797 (DCCXCVII) was a common year starting on Sunday (link will display the full calendar) of the Julian calendar. The denomination 797 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||1550|
|Balinese saka calendar||718–719|
|Chinese calendar||丙子年 (Fire Rat)|
3493 or 3433
— to —
丁丑年 (Fire Ox)
3494 or 3434
|- Vikram Samvat||853–854|
|- Shaka Samvat||718–719|
|- Kali Yuga||3897–3898|
|Japanese calendar||Enryaku 16|
|Minguo calendar||1115 before ROC|
|Seleucid era||1108/1109 AG|
|Thai solar calendar||1339–1340|
923 or 542 or −230
— to —
924 or 543 or −229
- April 19 – Empress Irene organizes a conspiracy against her son Constantine VI. He is captured and blinded; Irene exiles him to Principo, where he dies shortly thereafter of his wounds. Irene begins a 5-year reign, and calls herself basileus ("emperor") of the Byzantine Empire.
- King Charlemagne issues the Capitulare Saxonicum, making Westphalian, Angrian and Eastphalian Saxons equal to other peoples in the Frankish Kingdom. The Nordalbian Saxons revolt; a Frankish fleet is sent to the North Sea coast of Germany. It lands in Hadeln, a marshy coastal region between the Weser and Elbe estuaries, near modern-day Cuxhaven. Charlemagne invades northern Saxony, and again accepts the submission of the Saxons.
- Battle of Rhuddlan: Welsh forces, including those of Powys and Dyfed, clash with Mercians. King Coenwulf tries to re-assert his domination of northeast Wales. King Caradog ap Meirion of Gwynedd is killed during the fighting (approximate date).
- February 6 – Donnchad Midi, High King of Ireland
- Æthelberht of Whithorn, Anglo-Saxon bishop
- Al-Hasan ibn Qahtaba, Muslim military leader
- Bermudo I, king of Asturias (approximate date)
- Caradog ap Meirion, king of Gwynedd (approximate date)
- Constantine VI, emperor of the Byzantine Empire (b. 771)
- Cummascach mac Fogartaig, king of South Brega
- Guan Bo, chancellor of the Tang Dynasty (b. 719)
- Muireadhach mac Olcobhar, Anglo-Saxon abbot