The year 1661 in science and technology involved some significant events.

• Marcello Malpighi is the first to observe and correctly describe capillaries when he discovers them in a frog's lung.^[1]

• Robert Boyle's The Sceptical Chymist is published in London.

• John Evelyn's pamphlet Fumifugium is one of the earliest descriptions of air pollution.^[2]

• Abraham Cowley's pamphlet The Advancement of Experimental Philosophy.
• Johann Sperling's handbook Zoologia physica (posthumous).

• May 3 – Antonio Vallisneri, Italian physician and natural scientist (died 1730)
• December 18 – Christopher Polhem, Swedish scientist and inventor (died 1751)
• Guillaume François Antoine, Marquis de l'Hôpital, French mathematician (died 1704)
• approx. date – Alida Withoos, Dutch botanical artist (died 1730)

• Isaac Newton is admitted to Trinity College, Cambridge, as a sizar (June)

• October – Gérard Desargues, French geometer (born 1591)