Thomas Young FRS (13 June 1773 – 10 May 1829) was a British polymath who made notable contributions to the fields of vision, light, solid mechanics, energy, physiology, language, musical harmony, and Egyptology. He was instrumental in the decipherment of Egyptian hieroglyphs, specifically the Rosetta Stone.

Young has been described as "The Last Man Who Knew Everything". His work influenced that of William Herschel, Hermann von Helmholtz, James Clerk Maxwell, and Albert Einstein. Young is credited with establishing Christiaan Huygens' wave theory of light, in contrast to the corpuscular theory of Isaac Newton. Young's work was subsequently supported by the work of Augustin-Jean Fresnel.

Young belonged to a Quaker family of Milverton, Somerset, where he was born in 1773, the eldest of ten children. By the age of fourteen, Young had learned Greek, Latin, French, Italian, Syriac, Samaritan Hebrew, Arabic, Biblical Aramaic, Persian, Turkish, and Ge'ez.

Young began to study medicine in London at St Bartholomew's Hospital in 1792, moved to the University of Edinburgh Medical School in 1794, and a year later went to Göttingen, Lower Saxony, Germany, where he obtained the degree of doctor of medicine in 1796 from the University of Göttingen. In 1797 he entered Emmanuel College, Cambridge. In the same year he inherited the estate of his grand-uncle, Richard Brocklesby, which made him financially independent, and in 1799 he established himself as a physician at 48 Welbeck Street, London (now recorded with a blue plaque). Young published many of his first academic articles anonymously to protect his reputation as a physician.

In 1801, Young was appointed professor of natural philosophy (mainly physics) at the Royal Institution. In two years, he delivered 91 lectures. In 1802, he was appointed foreign secretary of the Royal Society, of which he had been elected a fellow in 1794. He resigned his professorship in 1803, fearing that its duties would interfere with his medical practice. His lectures were published in 1807 in the Course of Lectures on Natural Philosophy and contain a number of anticipations of later theories.

In 1811, Young became physician to St George's Hospital, and in 1814 he served on a committee appointed to consider the dangers involved in the general introduction of gas for lighting into London. In 1816 he was secretary of a commission charged with ascertaining the precise length of the seconds pendulum (the length of a pendulum whose period is exactly 2 seconds), and in 1818 he became secretary to the Board of Longitude and superintendent of the HM Nautical Almanac Office.

Young was elected a Foreign Honorary Member of the American Academy of Arts and Sciences in 1822. A few years before his death he became interested in life insurance, and in 1827 he was chosen as one of the eight foreign associates of the French Academy of Sciences. In the same year he became a first class corresponding member, living abroad, of the Royal Institute of the Netherlands. In 1828, he was elected a foreign member of the Royal Swedish Academy of Sciences.

In 1804, Young married Eliza Maxwell. They had no children.