|644 by topic|
|Ab urbe condita||1397|
|Balinese saka calendar||565–566|
|Chinese calendar||癸卯年 (Water Rabbit)|
3340 or 3280
— to —
甲辰年 (Wood Dragon)
3341 or 3281
|- Vikram Samvat||700–701|
|- Shaka Samvat||565–566|
|- Kali Yuga||3744–3745|
|Minguo calendar||1268 before ROC|
|Seleucid era||955/956 AG|
|Thai solar calendar||1186–1187|
770 or 389 or −383
— to —
771 or 390 or −382
Year 644 (DCXLIV) was a leap year starting on Thursday (link will display the full calendar) of the Julian calendar. The denomination 644 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.
- Valentinus, Byzantine general, attempts to usurp the throne of his son-in-law Constans II. He appears at the gates of Constantinople with a contingent of Byzantine troops, and demands to be crowned emperor. His claim is rejected, and Valentinus is lynched by the populace.
- Oswine, son of the late king Osric of Deira, manages, despite armed objections from King Oswiu of Bernicia, to establish himself as king of Deira (Northern England). His succession, probably the choice of the people of Deira, splits the Kingdom of Northumbria.
- November 6 – Caliph Umar, age 65, dies of wounds inflicted on November 3 by the Persian slave Piruz Nahavandi at Medina, after a 10-year reign. On his death bed he appoints a committee to determine his successor. They select Uthman ibn Affan, who becomes caliph of the Rashidun Caliphate.
- Emperor Taizong of the Tang Dynasty sends a Chinese expeditionary force, to invade and annex the Tarim Basin kingdom of Karasahr in Xinjiang, a vassal of the Western Turkic Khaganate. The oasis state is conquered, and Western Turks sent to assist Karasahr are defeated by the Tang forces.
- Zenkoji, a Buddhist temple and landmark spot in Nagano, Japan, officially built by Empress Kogyoku.
- January 17 – Sulpitius the Pious, French bishop and saint
- October 10 – Paulinus, Archbishop of York
- Otto, mayor of the palace (Austrasia)
- Radulf, king of Thuringia (approximate date)
- Trudpert, Irish missionary (or 607)
- Umar I, Muslim caliph (b. 579)
- Valentinus, Byzantine general and usurper