1781 (MDCCLXXXI) was a common yearstarting on Monday of the Gregorian calendarand a common year starting on Friday of the Julian calendar, the 1781st year of the Common Era (CE) and Anno Domini (AD) designations, the 781st year of the 2ndmillennium, the 81st year of the 18thcentury, and the 2nd year of the 1780s decade. As of the start of 1781, the Gregorian calendar was 11 days ahead of the Julian calendar, which remained in localized use until 1923.

• March 13 – Uranus.
• End of the American Revolutionary War

• January 26 – Achim von Arnim, German writer (d. 1831)
• January 30 – Adelbert von Chamisso, German writer (d. 1838)
• February 17 – Rene Theophile Hyacinthe Laennec, French physician and inventor (d. 1826)
• March 4 – Rebecca Gratz, American educator and philanthropist (d. 1869)
• March 13 – Karl Friedrich Schinkel, German architect and painter (d. 1841)
• June 9– George Stephenson, English engineer (d. 1848)
• June 21 – Siméon-Denis Poisson, French mathematician and physicist (d. 1840)
• July 6 – Thomas Stamford Raffles, English founder of Singapore (d. 1826)
• July 6 – John D. Sloat, American naval officer (d. 1867)
• July 27 – Mauro Giuliani, Italian composer (d. 1828)
• September 3 – Eugène de Beauharnais, French nobleman, son of Napoleon's wife Josephine (d. 1824)
• September 6 – Anton Diabelli, Austrian music publisher, editor, and composer (d. 1858)
• October 1 – James Lawrence, U.S. Navy officer (d. 1813)
• November 6 – Lucy Aikin, English writer (d. 1864)
• November 20 – Karl Friedrich Eichhorn, German jurist (d. 1854)
• November 29 – Andrés Bello, Venezuelan poet, lawmaked, teacher, philosopher and sociologist (d. 1865)
• November 30 – Alexander Berry, Scottish adventurer and Australian pioneer (d. 1873)
• December 11 – Sir David Brewster, Scottish physicist (d. 1868)
• William Williams of Wern, minister (d. 1840)