Stephen Hales (September 17, 1677 - January 4, 1761) was an English physiologist, chemist and inventor.

Hales studied the role of air and water in the maintenance of both plant and animal life. He gave accurate accounts of the movements of water in plants, and demonstrated that plants absorbed air. He discovered the dangers of breathing in stale air, and invented a ventilator which improved survival rates when employed on ships, in hospitals and in prisons.

Hales was born at Bekesbourne in Kent. In June 1696 he was entered as a pensioner of Corpus Christi College, Cambridge, with the view of taking holy orders, and in February 1703 was admitted to a fellowship. In 1708 Hales was presented to the perpetual curacy of Teddington in Middlesex, where he remained all his life, notwithstanding that he was subsequently appointed rector of Porlock in Somerset, and later of Faringdon in Hampshire. In 1717 he was elected fellow of the Royal Society, which awarded him the Copley Medal in 1739. In 1732 he was named one of a committee for establishing a colony in Georgia, and the next year he received the degree of doctor of divinity from Oxford. He was appointed almoner to the princess dowager of Wales in 1750. On the death of Sir Hans Sloane in 1753, Hales was chosen foreign associate of the French Academy of Sciences.

Known as a pioneer of experimental physiology, Hales showed that some reflexes are mediated by the spinal cord.

Hales is best known for his Statical Essays. The first volume, Vegetable Staticks (1727), contains an account of numerous experiments in plant-physiology - the loss of water in plants by evaporation, the rate of growth of shoots and leaves, and variations in root-force at different times of the day. The second volume (1733) on Haemastaticks, containing experiments on the "force of the blood" in various animals, its rate of flow, and the capacity of the different vessels.

Hales also studied stones taken from the bladder and kidneys and suggested solvents which might reduce them without surgery. He also invented the surgical forceps.