Year 645 (DCXLV) was a common year starting on Saturday (link will display the full calendar) of the Julian calendar. The denomination 645 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||1398|
|Balinese saka calendar||566–567|
|Chinese calendar||甲辰年 (Wood Dragon)|
3341 or 3281
— to —
乙巳年 (Wood Snake)
3342 or 3282
|- Vikram Samvat||701–702|
|- Shaka Samvat||566–567|
|- Kali Yuga||3745–3746|
|Japanese calendar||Taika 1|
|Minguo calendar||1267 before ROC|
|Seleucid era||956/957 AG|
|Thai solar calendar||1187–1188|
771 or 390 or −382
— to —
772 or 391 or −381
- Alexandria revolts against Arab rule at the appearance of a Byzantine fleet, (300 ships) and Byzantine forces recapture the city. Abdullah ibn Sa'ad, Arab governor of Egypt, mounts an assault and retakes it. He begins building a Muslim fleet.
- Plato, exarch (imperial governor) of Ravenna, invades the southern Po Valley. The Lombards under King Rothari defeat him on the banks of the Panaro River (near Modena); 8,000 imperial troops are killed.
- King Cenwalh of Wessex (according to Bede) is driven from his kingdom by his brother-in-law, King Penda of Mercia. He flees to the court of king Anna of East Anglia, and is baptised while in exile. Penda overruns Wessex.
- Gwynedd and much of Wales is in the grasp of famine. Would-be king Cadwaladr Fendigaid flees to Brittany. Civil war continues in his kingdom (approximate date).
- July 10 – Isshi Incident: Prince Naka-no-Ōe and Fujiwara no Kamatari assassinate Soga no Iruka, during a coup d'état at the imperial palace.
- Empress Kōgyoku is forced to abdicate the throne in favor of her younger brother Kōtoku, age 49, who becomes the 36th emperor of Japan.
- Naka-no-Ōe becomes crown prince and prime minister. Supporters of the semi-legendary regent Prince Shōtoku gain supremacy in Japan.
- Emperor Kōtoku establishes the Taika Reform: a land reform based on Confucian ideas and philosophies from China (approximate date).
- Kōtoku creates a new city at Naniwa, and moves the capital from Yamato Province. The capital has a sea port, establishing foreign trade and diplomatic relations.
- Goguryeo–Tang War: A Chinese expeditionary army under Emperor Taizong crosses the Liao River into Goguryeo (One of the Three kingdoms of Korea).
- July 18 – Tang forces under Li Shiji heading southeast, toward the Yalu River, put the strategic fortress city of Ansi (Liaoning) under siege.
- September – Taizong is unable to capture Ansi fortress defended by Korean general Yang Manchun. Food supplies running low, he withdraws his forces.
- Xuanzang, Chinese Buddhist monk, returns to China after a 16-year pilgrimage to India. He is greeted with much honor by Emperor Taizong.
- The Giant Wild Goose Pagoda at Ci'en Temple, Xi'an (Shanxi) is first erected during the Tang Dynasty (approximate date).
- April 26 – Richarius, Frankish hermit and monk
- July 10 – Soga no Iruka, statesman of Japan
- October 21 – Zhenzhu Khan, khan of Xueyantuo
- Al-Khansa, Arabic poet (b. 575)
- Cen Wenben, chancellor and editor of the Book of Zhou (b. 595)
- Li Chengqian, crown prince of the Tang Dynasty
- Soga no Emishi, statesman of Japan (b. 587)
- Yan Shigu, Chinese author of the Tang Dynasty (b. 581)
- The Caliphate Its Rise, Decline and Fall. Conquest of Egypt, Chapter XXII (p. 166)
- Graff, David A. (2002). "Medieval Chinese Warfare, 300–900". London: Routledge, p. 197. ISBN 9780415239554
- Lee, Kenneth B. (1997). Korea and East Asia: "The story of a phoenix". Westport: Praeger. p. 16. ISBN 9780275958237.