August 12 is the 224th day of the year (225th in leap years). There are 141 days remaining until the end of the year.
- 1503: King Christian III of Denmark and Norway (d. 1559)
- 1566: Infanta Isabella Clara Eugenia of Spain (d. 1633)
- 1629: Tsar Alexis of Russia (d. 1676)
- 1762: King George IV of the United Kingdom (d. 1830)
- 1872: Princess Marie Louise of Schleswig-Holstein (d. 1956)
- 1904: Alexei Nikolaevich, Tsarevich of Russia (d. 1918)
- 1932: Sirikit, Queen consort of Thailand, wife of Bhumibol Adulyadej (Rama IX)
- 2000: Prince Achileas-Andreas of Greece and Denmark
- 30 BC: Cleopatra VII (b. 69 BC)
- 875: Louis II of Italy, Holy Roman Emperor (b. 825)
- 1424: Yongle Emperor of China (b. 1360)
- 1648: Ibrahim I, Ottoman Sultan (b. 1615)
- 1778: Peregrine Bertie, 3rd Duke of Ancaster and Kesteven (b. 1714)
- 1822: Robert Stewart, Viscount Castlereagh (b. 1769)
- 1941: 1st Marquess of Willingdon (b. 1866)
- 30 BC: Cleopatra VII Philopator, the last ruler of the Egyptian Ptolemaic dynasty, commits suicide, allegedly by means of an asp bite.
- 1099: First Crusade: Battle of Ascalon Crusaders under the command of Godfrey of Bouillon defeat Fatimid forces led by Al-Afdal Shahanshah. This is considered the last engagement of the First Crusade.
- 1121: Battle of Didgori: the Georgian army under King David the Builder wins a decisive victory over the famous Seljuk commander Ilghazi.
- 1164: Battle of Harim: Nur ad-Din Zangi defeats the Crusader armies of the County of Tripoli and the Principality of Antioch.
- 1624: The president of Louis XIII of France's royal council is arrested, leaving Cardinal Richelieu in the role of the King's principal minister.
- 1687: Battle of Mohács: Charles of Lorraine defeats the Ottomans.
- 1831: French intervention forces William I of the Netherlands to abandon his attempt to suppress the Belgian Revolution.