James Watt (19 January 1736 – 19 August 1819) was a Scottish inventor and engineer whose improvements to the steam engine were fundamental to the changes wrought by the Industrial Revolution.

James Watt was born on 19 January 1736 in Greenock. Watt was saved from this impasse by the professors of the University of Glasgow, who offered him the opportunity to set up a small workshop within the university. It was established in 1757 and one of the professors, the physicist and chemist Joseph Black, became Watt's friend and mentor.
It failed to work satisfactorily, but he continued his experiments and began to read everything about it he could. He independently discovered the importance of latent heat in understanding the engine, which, unknown to him, Black had famously discovered some years before. He learned that the University owned a model Newcomen engine, but it was in London for repairs. Watt got the university to have it returned, and he made the repairs in 1763. It too just barely worked, and after much experimentation he showed that about 80% of the heat of the steam was consumed in heating the cylinder, because the steam in it was condensed by an injected stream of cold water. His critical insight, to cause the steam to condense in a separate chamber apart from the piston, and to maintain the temperature of the cylinder at the same temperature as the injected steam, came finally in 1765 and he soon had a working model.
Finally, in 1776 the first engines were installed and working in commercial enterprises. These first engines were used for pumps and produced only reciprocating motion. Orders began to pour in and for the next five years Watt was very busy installing more engines, mostly in Cornwall for pumping water out of mines.
 A double acting engine, in which the steam acted alternately on the two sides of the piston was one. A throttle valve to control the power of the engine, and a centrifugal governor to keep it from "running away" were very important. He described methods for working the steam expansively. A compound engine, which connected two or more engines was described. Two more patents were granted for these in 1781 and 1782.
 Another important invention, one of which Watt was most proud, was the three-bar linkage which produced the straight line motion required for the cylinder rod and pump, from the connected rocking beam, whose end moves in a circular arc. This was patented in 1784. These improvements taken together produced an engine which was something like five times as efficient in its use of fuel as the Newcomen engine.

The SI unit of power, the watt, is named after him.