The year 1781 in science and technology involved some significant events.

• March 13 – William Herschel observes Uranus (although initially recording it as a comet).^[1]
• March 20 – Pierre Méchain discovers dwarf galaxy NGC 5195.
• Charles Messier's final catalogue of Messier objects is published.
• Christian Mayer's catalogue of binary stars is published.^[2]

• Felice Fontana uses a microscope to describe the axon of a brain cell.
• John Latham begins publication of A General Synopsis of Birds.

• Archibald Cochrane, 9th Earl of Dundonald is granted a British patent for the manufacture of coal tar.^[3][4]
• Carl Wilhelm Scheele ascertains that a new acid can be made from tungstenite, leading to the discovery of tungsten in 1783.^[5]
• Autumn – Peter Jacob Hjelm isolates molybdenum.^[5][6]

Hendry Carvendish proved that water is a compound is made up of two elements that is two molecule hydrogen and one molecule of oxygen are combined in fixed ratio (2:1)water is a compound

• Copley Medal: William Herschel^[7]

• January 30 – Adelbert von Chamisso, poet and botanist (died 1838)
• February 17 – Rene Theophile Hyacinthe Laennec, French physician, inventor of the stethoscope (died 1826).^[8]
• May 29 – John Walker, English chemist (died 1859)
• June 9 – George Stephenson, English locomotive engineer (died 1848)
• June 21 – Siméon Poisson, mathematician (died 1840)
• July 6 – Thomas Stamford Raffles, founder of the Zoological Society of London (died 1826)
• September 14 – James Walker, Scottish civil engineer (died 1862)
• October 5 – Bernhard Bolzano, mathematician (died 1848)
• December 11 – David Brewster, Scottish physicist (died 1868)

• May 27 – Giovanni Battista Beccaria, Italian physicist (born 1716)