Sir James Dewar FRS (20 September 1842 – 27 March 1923) was a Scottish chemist and physicist. He is probably best-known today for his invention of the Dewar flask, which he used in conjunction with extensive research into the liquefaction of gases. He was also particularly interested in atomic and molecular spectroscopy, working in these fields for more than 25 years.

James Dewar was born in Kincardine-on-Forth in 1842, the youngest of six boys. He lost his parents at the age of 15. He was educated at Dollar Academy and the University of Edinburgh, where he studied under Lord Playfair, and later became Lord Playfair's assistant. Dewar would also study under August Kekulé at Ghent.

In 1875, Dewar was elected Jacksonian professor of natural experimental philosophy at the University of Cambridge, becoming a fellow of Peterhouse. He became a member of the Royal Institution and later, replaced Dr. John Hall Gladstone in the role of Fullerian Professor of Chemistry in 1877. Dewar was also the President of the Chemical Society in 1897 and the British Association for the Advancement of Science in 1902, as well as serving on the Royal Commission established to examine London's water supply from 1893 to 1894 and the Committee on Explosives. It was whilst he was serving on the Committee on Explosives that he and Frederick Augustus Abel developed cordite, a smokeless gunpowder alternative.