|Year:||1837 1838 1839 – 1840 – 1841 1842 1843|
1840 (MDCCCXL) was a leap year starting on Wednesday of the Gregorian calendar (or a leap year starting on Monday of the 12-day slower Julian calendar).
- January 19 – Captain Charles Wilkes circumnavigates Antarctica, claiming what becomes known as Wilkes Land for the United States.
- July 4 – The Cunard Line's 700-ton wooden paddlewheel steamer RMS Britannia departs from Liverpool, bound for Halifax, Nova Scotia on the first transatlantic passenger cruise.
- November 4 – U.S. presidential election, 1840: William Henry Harrison defeats Martin Van Buren.