Sir David Brewster (11 December 1781 – 10 February 1868) was a Scottish scientist, inventor, author, and academic administrator. In science he is principally remembered for his experimental work in physical optics, mostly concerned with the study of the polarization of light and including the discovery of Brewster's angle. He studied the birefringence of crystals under compression and discovered photoelasticity, thereby creating the field of optical mineralogy. For this work, William Whewell dubbed him the "father of modern experimental optics" and "the Johannes Kepler of optics."

Brewster was a pioneer in photography. He invented an improved stereoscope, which he called "lenticular stereoscope" and which became the first portable 3D-viewing device. He also invented the stereoscopic camera, two types of polarimeters, the polyzonal lens, the lighthouse illuminator, and the kaleidoscope.

Brewster was a devout Presbyterian and marched arm-in-arm with his brother during the events of the Disruption of 1843, which led to the formation of the Free Church of Scotland. As a historian of science, Brewster focused on the life and work of his hero, Isaac Newton. Brewster published a detailed biography of Newton in 1831 and later became the first scholar to examine many of the papers in Newton's Nachlass (the unpublished documents left after his death). Brewster also wrote numerous works of popular science, and was one of the founders of the British Science Association, of which he was elected president in 1849. He became the public face of higher education in Scotland, serving as Principal of the University of St Andrews (1837–1859) and later of the University of Edinburgh (1859–1868). Brewster also edited the 18-volume Edinburgh Encyclopædia.

David Brewster was born in the Canongate in Jedburgh, Roxburghshire, to Margaret Key (1753–1790) and James Brewster (c. 1735–1815), the rector of Jedburgh Grammar School and a teacher of high reputation. David was the third of six children, two daughters and four sons: James (1777–1847), minister at Craig, Ferryden; David; George (1784–1855), minister at Scoonie, Fife; and Patrick (1788–1859), minister at the abbey church, Paisley.

At the age of 12, David Brewster matriculated at the University of Edinburgh with the intention of becoming a clergyman. He received his MA in 1800, was licensed as a minister of the Church of Scotland, and then preached around Edinburgh on several occasions. By then, Brewster had already shown a strong inclination for the natural sciences and had established a close association with James Veitch of Inchbonny. Veitch, who enjoyed a local reputation as a man of science and was particularly skilled in making telescopes, was characterized by Sir Walter Scott as a "self-taught philosopher, astronomer and mathematician".

Brewster is buried in the grounds of Melrose Abbey, in Roxburghshire.

Though Brewster duly finished his theological studies and was licensed to preach, his other interests distracted him from the duties of his profession. In 1799 fellow-student Henry Brougham persuaded him to study the diffraction of light. The results of his investigations were communicated from time to time in papers to the Philosophical Transactions of London and other scientific journals. The fact that other scientists – notably Étienne-Louis Malus and Augustin Fresnel – were pursuing the same investigations contemporaneously in France does not invalidate Brewster's claim to independent discovery, even though in one or two cases the priority must be assigned to others. A lesser-known classmate of his, Thomas Dick, also went on to become a popular astronomical writer.

The most important subjects of his inquiries can be enumerated under the following five headings: