| January 11 | Monroe and Livingston sail for Paris to buy New Orleans; they buy La |
| February 4 | William Dunlap, adapts French melodrama "Voice of Nature" |
| February 14 | Apple parer patented by Moses Coats, Downington, Pennsylvania |
| February 19 | Congress accepts Ohio's constitution, statehood not ratified till 1953 |
| February 24 | Supreme Court 1st rules a law unconstitutional (Marbury vs. Madison) |
| February 25 | 1,800 sovereign German states unite into 60 states |
| February 27 | Great fire in Bombay, India |
| March 1 | Ohio becomes 17th state |
| March 3 | 1st impeachment trial of a federal judge, John Pickering, begins |
| March 19 | Johann von Schillers "Die Braut von Messina," premieres in Weimar |
| April 1 | French law rules the use of intention |
| April 5 | 1st performance of Beethoven's 2nd Symphony in D |
| April 26 | Meteorites fall in L'Aigle, France |
| April 30 | U.S. doubles in size through Louisiana Purchase |
| May 16 | Peace of Amiens ends |
| May 17 | John Hawkins and Richard French patent the Reaping Machine |
| May 18 | Britain declares war on France after Napoleon Bonaparte continues interfering in Italy and Switzerland |
| May 22 | 1st public library opens (Connecticut) |
| July 23 | Robert Emmett's insurrected in Dublin |
| August 9 | 1st horses arrive in Hawaii |
| September 23 | Battle of Assaye-British-Indian forces beat Maratha Army |
| October 20 | U.S. Senate ratifies Louisiana Purchase |
| November 18 | Battle of Vertieres, in which Haitians defeat French |
| November 29 | Dessalines and Christophe declare St. Domingue (Haiti) independent |
| November 30 | Spain cedes her claims to Louisiana Territory to France |
| December 20 | Louisiana Purchase formally transferred from France to U.S. for $27M |
