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|999 by topic|
|Ab urbe condita||1752|
|Balinese saka calendar||920–921|
|Chinese calendar||戊戌年 (Earth Dog)
3695 or 3635
— to —
己亥年 (Earth Pig)
3696 or 3636
|- Vikram Samvat||1055–1056|
|- Shaka Samvat||920–921|
|- Kali Yuga||4099–4100|
|Japanese calendar||Chōtoku 5 / Chōhō 1
|Minguo calendar||913 before ROC
|Seleucid era||1310/1311 AG|
|Thai solar calendar||1541–1542|
1125 or 744 or −28
— to —
1126 or 745 or −27
|Wikimedia Commons has media related to 999.|
Year 999 (CMXCIX) was a common year starting on Sunday (link will display the full calendar) of the Julian calendar. Depending on how one defines a millennium, 999 was either the last or second-to-last year of the first millennium.
- Fall – King Bermudo II abdicates in favor of his 5-year-old son Alfonso V as ruler of León. Moorish invaders have forced Bermudo to recognize the suzerainty of their leader, Umayyad vizier and the de facto ruler Al-Mansur.
- December 30 - Battle of Glenmama: The combined forces of Munster and Meath under Brian Boru (High King of Ireland) inflict a crushing defeat on the allied armies of Leinster and Dublin near Lyons Hill (County Kildare).
- The Karakhanids invade from north of the Syr Darya River which ends the Samanid Empire (modern Iran). The Samanid domains are split up between the Ghaznavid Dynasty and the Karakhanids.
- Bjarni Herjólfsson, Norse-Icelandic trader and explorer, is blown far off course to the southwest while trying to sail from Iceland to Greenland. He sights (as the first European discoverer) land that will prove to be part of the mainland of the Americas (approximate date).
- February 18 – Pope Gregory V dies after a 3-year pontificate in which he has been forced to flee Rome by the Cresenti family. He is succeeded by Sylvester II as the 139th pope of the Catholic Church.
- Bishop Aldhun consecrates a cathedral (later Durham Cathedral) at the location where the remains of St. Cuthbert are relocated. The remains from Lindisfarne are moved of the danger of Viking raids.
- Sigmundur Brestisson, an Viking chieftain, introduces Christianity in the Faroe Islands.
- April 11 – Bao Zheng, politician of the Song Dynasty (d. 1062)
- Berengar of Tours, French theologian (approximate date)
- Fujiwara no Ishi, Japanese empress consort (d. 1036)
- Odo of Rennes, duke and regent of Brittany (d. 1079)
- February 7 – Boleslaus II (the Pious), duke of Bohemia
- February 18 – Gregory V, pope of the Catholic Church
- June 11 – Ebergar (or Everger), archbishop of Cologne
- November 4 – Gregor von Burtscheid, German abbot
- November 29 – Li Chun'an, Chinese merchant (b. 921)
- December 16 – Adelaide, empress regent of the Holy Roman Empire (b. 931)
- Alfred of Malmesbury (or Aelfric), English abbot and writer
- Cao Bin, Chinese general and governor (jiedushi) (b. 931)
- Ceallach ua Maílcorgus, Irish chief poet of Connacht
- Maredudd ab Owain, king of Gwynedd (Wales) (approximate date)
- Matilda, German princess-abbess and daughter of Otto I (b. 955)
- Muirgheas mac Aedh, king of Uí Díarmata (Ireland)
- Subh of Córdoba, mother and regent of Hisham II
- Yelü Xiezhen, Chinese general and politician