1840 (MDCCCXL) was a leap year starting on Wednesday of the Gregorian calendar and a leap year starting on Monday of the Julian calendar, the 1840th year of the Common Era (CE) and Anno Domini (AD) designations, the 840th year of the 2nd millennium, the 40th year of the 19th century, and the 1st year of the 1840s decade. As of the start of 1840, the Gregorian calendar was 12 days ahead of the Julian calendar, which remained in localized use until 1923.

• The Great Natchez Tornado: A massive tornado hits Natchez, Mississippi. Before it was over, 317 people had died and 209 were injured.

• January 3– Father Damien, priest
• January 18 – Edmund Barton, 1st Prime Minister of Australia (d. 1920)
• May 7 – Pyotr Ilyich Tchaikovsky, Russian composer
• Kuroda Kiyotaka, Prime Minister of Japan

• May 27 – Niccolò Paganini, Italian violinist and composer