The year 1805 in science and technology included many events, some of which are listed here.

Exploration

• April 7 - The Lewis and Clark expedition leaves Fort Mandan, bound for the Pacific Ocean
• August 9 - Zebulon Pike leaves St. Louis to explore the headwaters of the Mississippi River
• November 16 - The Lewis and Clark expedition reaches the Pacific Ocean at the mouth of the Columbia River

Awards

• Copley Medal: Humphry Davy

Births

• February 13 - Peter Gustav Lejeune Dirichlet, mathematician (died 1859)
• May 12 - William Rowan Hamilton, mathematician, physicist, and astronomer (died 1865)
• December 16 - Isidore Geoffroy Saint-Hilaire, zoologist (died 1861)
• December 20 - Thomas Graham, chemist (died 1869)

Deaths

• January 23 - Claude Chappe, inventor of the semaphore system (born 1763)
• December 23 - Pehr Osbeck , Swedish botanist and explorer, pupil of Linnaeus (born 1723)