1869 in science
- November 4 – The first issue of scientific journal Nature is published in London, edited by Norman Lockyer.
- March 6 – Dmitri Mendeleev makes a formal presentation of his periodic table to the Russian Chemical Society.
- June 15 – John Wesley Hyatt patents celluloid, in Albany, New York.
- German chemist Lothar Meyer makes in his work „Die Natur der chemischen Elemente als Funktion ihrer Atomgewichte" a formal presentation of his revised and expanded version of his 1864 table independently periodic table.
- Hippolyte Mège-Mouriès patents margarine (as oleomargarine) in France as a beef tallow and skimmed milk substitute for butter.
- Publication of Adolphe Wurtz's Dictionnaire de chimie pure et appliquée begins in Paris.
- April 6 – The American Museum of Natural History is founded in New York.
- Paul Langerhans discovers the pancreatic islets.
- Friedrich Miescher discovers deoxyribonucleic acid (DNA) in the pus of discarded surgical bandages. Found in the nuclei of cells, Miescher names it "nuclein".
- French missionary and naturalist Père Armand David receives the skin of a giant panda from a hunter, the first time this species has become known to a Westerner; he also first describes a specimen of the "pocket handkerchief tree", which will be named in his honor as Davidia involucrata.
- Alfred Russel Wallace publishes The Malay Archipelago.
- W. Stanley Jevons publishes The Substitution of Similars and has a "Logic Piano" constructed to work out problems in symbolic logic.
- Hermann Schwarz devizes Schwarz–Christoffel mapping.
- February 14 – C. T. R. Wilson (died 1959), Scottish winner of the Nobel Prize in Physics.
- April 8 – Harvey Cushing (died 1939), American neurosurgeon.
- July 18 – Maria von Linden (died 1936), German bacteriologist and zoologist.
- August 23 – Robert Gunther (died 1940), English historian of science.
- October 3 – Robert W. Paul (died 1943), English pioneer of cinematography.
- December 16 – Bertha Lamme (died 1943), American electrical engineer.
- Helen Boyle (died 1957), British physician and psychologist.
- July 22 – John A. Roebling (born 1806), German American bridge engineer.
- September 11 – Thomas Graham (born 1805), Scottish chemist.