1040 (MXL) was a leap year starting on Tuesday of the Julian calendar, the 1040th year of the Common Era (CE) and Anno Domini (AD) designations, the 40th year of the 2nd millennium, the 40th year of the 11th century, and the 1st year of the 1040s decade. As of the start of 1040, the Gregorian calendar was 6 days ahead of the Julian calendar, which was the dominant calendar of the time.

• March War of Independence of Western Xia occurred.
• June 17 – Harthacanute lands at Sandwich and reclaims the English Throne.
• August 15 – King Duncan I is killed in battle against his first cousin and rival Macbeth. The latter succeeds him as Scottish Monarch.
• The oldest brewery still active is founded at Weihenstephan.
• Seljuk Turks defeat the Ghaznavids at the Battle of Dandanaqan.

• February 22 – Rashi, French rabbi and commentator (d. 1105)
• June 27 – King Ladislaus I of Hungary (d. 1095)
• King Alfonso VI of Castile, Spanish royal (died 1109)

• March 17 – Harold Harefoot, King of England
• August 15 – King Duncan I of Scotland (b. 1001)
• October 1 – Alan III, Duke of Brittany (poisoned) (b. 997)

• For the U.S. Internal Revenue Service (IRS) form, see Form 1040.