May 15 is the 135th day of the year (136th in leap years) in the Gregorian calendar. There are 230 days remaining until the end of the year.
- 1773: Prince Klemens von Metternich (d. 1859)
- 1945: Duarte Pio, Duke of Braganza
- 1957: Prince Jean of Luxembourg
- 1957: Princess Margaretha of Liechtenstein
- 1981: Zara Phillips
- 2003: Anna Maria Morales y de Grecia
- 392: Valentinian II (b. 371)
- 1036: Emperor Go-Ichijō of Japan (b. 1008)
- 1157: Yuri Dolgorukiy
- 1174: Nur ad-Din Zangi (b. 1118)
- 1470: Charles VIII of Sweden (b. 1409)
- 1591: Dmitry Ivanovich (b. 1582)
- 392: Emperor Valentinian II is assassinated while advancing into Gaul against the Frankish usurper Arbogast. He is found hanging in his residence at Vienne.
- 1536: Anne Boleyn, Queen of England, stands trial in London on charges of treason, adultery and incest. She is condemned to death by a specially-selected jury.
- 1567: Mary, Queen of Scots, marries James Hepburn, 4th Earl of Bothwell, her third husband.
- 1800: George III of the United Kingdom survives an assassination attempt by James Hadfield, who is later acquitted by reason of insanity.