|721 by topic|
|Ab urbe condita||1474|
|Balinese saka calendar||642–643|
|Chinese calendar||庚申年 (Metal Monkey)|
3417 or 3357
— to —
辛酉年 (Metal Rooster)
3418 or 3358
|- Vikram Samvat||777–778|
|- Shaka Samvat||642–643|
|- Kali Yuga||3821–3822|
|Japanese calendar||Yōrō 5|
|Minguo calendar||1191 before ROC|
|Seleucid era||1032/1033 AG|
|Thai solar calendar||1263–1264|
847 or 466 or −306
— to —
848 or 467 or −305
Year 721 (DCCXXI) was a common year starting on Wednesday (link will display the full calendar) of the Julian calendar. The denomination 721 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.
- February 13 – King Chilperic II dies at Attigny (Ardennes), after a five-year reign. He is succeeded by Theuderic IV, infant son of Dagobert III, as Merovingian ruler of the Franks, under the control of the mayor of the palace, Charles Martel.
- Summer – Charles Martel restores the authority of the Austrian palace throughout the Frankish Kingdom, including against Frankish-claimed Aquitaine and Provence (Southern France). He exiles Rigobert, bishop of Reims, to Gascony.
- June 9 – Battle of Toulouse: After besieging Toulouse for three months, Muslim forces under governor (wali) Al-Samh ibn Malik al-Khawlani are defeated by Eudes, duke of Aquitaine, preventing the extension of Umayyad control over Gaul.
- Anbasa ibn Suhaym Al-Kalbi is appointed governor of Al-Andalus, after the death of Al-Samh. The Muslims under Abdul Rahman al-Ghafiqi withdraw to Narbonne. The Visigothic duke Amrus of the Lerida area recognises Umayyad rule.
- Tervel, ruler (khagan) of the Bulgarian Empire, dies after a 21-year reign. He is succeeded by Kormesiy, possibly a son of Tervel, who is co-ruler and a descendant of the royal Dulo clan.
- Prague is founded (according to legend) by Princess Libuše and her husband Přemysl, founder of the Přemyslid dynasty (approximate date).
- King Ine of Wessex defeats Prince Cynewulf, an unknown relation making a push for the throne of Wessex.
- Rains and heavy storms around the southern seaport of Yangzhou destroy over 1,000 ships and boats in the Grand Canal, during the Tang Dynasty (approximate date).
- Prüm Abbey is founded by Bertrada, daughter of former king Theuderic III, and her son Charibert, count of Laon (approximate date).
- Abul Abbas al-Saffah, Muslim caliph (approximate date)
- Fujiwara no Uona, Japanese minister (d. 783)
- Jābir ibn Hayyān, Muslim alchemist (approximate date)
- Tachibana no Naramaro, Japanese statesman (d. 757)
- February 13 – Chilperic II, king of the Franks
- May 7 – John of Beverley, bishop of York
- December 29 – Genmei, empress of Japan (b. 660)
- Al-Samh ibn Malik al-Khawlani, Muslim general (approximate date)
- Ardo, king of the Visigoths (or 720)
- Eadfrith, bishop of Lindisfarne
- Headda, bishop of Lichfield (approximate date)
- Liu Zhiji, Chinese historian (b. 661)
- Tervel, ruler (khagan) of the Bulgarian Empire
- Yao Chong, chancellor of the Tang Dynasty (b. 650)