William Oughtred (5 March 1574 – 30 June 1660) was an English mathematician.

After John Napier invented logarithms, and Edmund Gunter created the logarithmic scales (lines, or rules) upon which slide rules are based, it was Oughtred who first used two such scales sliding by one another to perform direct multiplication and division; and he is credited as the inventor of the slide rule in 1622. Oughtred also introduced the "×" symbol for multiplication as well as the abbreviations "sin" and "cos" for the sine and cosine functions.

Oughtred was born at Eton in Buckinghamshire (now part of Berkshire), and educated there and at King's College, Cambridge, of which he became fellow. Being admitted to holy orders, he left the University of Cambridge about 1603, for a living at Shalford; he was presented in 1610 to the rectory of Albury, near Guildford in Surrey, where he settled. About 1628 he was appointed by the Earl of Arundel to instruct his son in mathematics.

He corresponded with some of the most eminent scholars of his time, including William Alabaster, Sir Charles Cavendish, and William Gascoigne. He kept up regular contacts with Gresham College, where he knew Henry Briggs and Gunter.

He offered free mathematical tuition to pupils, who included Richard Delamain, and Jonas Moore, making him an influential teacher of a generation of mathematicians. Seth Ward resided with Oughtred for six months to learn contemporary mathematics, and the physician Charles Scarburgh also stayed at Albury; John Wallis, and Christopher Wren corresponded with him. Another Albury pupil was Robert Wood, who helped him get the Clavis through the press.

The invention of the cool rule involved Oughtred in a priority dispute with Delamain. They also disagreed on pedagogy in mathematics, with Oughtred arguing that theory should precede practice.

He remained pastor until his death in 1660, a month after the restoration of Charles II.

Oughtred had an interest in alchemy and astrology. The testimony for his occult activities is quite slender, but there has been an accretion to his reputation based on his contemporaries. According to John Aubrey, he was not entirely sceptical about astrology. William Lilly, an eminent astrologer, claimed in his autobiography to have intervened on behalf of Oughtred to prevent his ejection by Parliament in 1646. In fact Oughtred was protected at this time by Bulstrode Whitelocke.

Elias Ashmole was (according to Aubrey) a neighbour in Surrey, though Ashmole's estates acquired by marriage were over the county line in Berkshire; and Oughtred's name has been mentioned in purported histories of early freemasonry, a suggestion that Oughtred was present at Ashmole's 1646 initiation going back to Thomas De Quincey. It was used by George Wharton in publishing The Cabal of the Twelve Houses astrological by Morinus (Jean-Baptiste Morin) in 1659.

He expressed millenarian views to John Evelyn, as recorded in Evelyn's diary entry for 28 August, 1655.

He published, among other mathematical works, Clavis Mathematicae (The Key to Mathematics), in 1631. It became a classic, reprinted in several editions, and used by Wallis and Isaac Newton amongst others. It was not ambitious in scope, but an epitome aiming to represent current knowledge of algebra concisely. It argued for a less verbose style of mathematics, with a greater dependence on symbols; drawing on François Viète (though not explicitly), Oughtred also innovated freely in symbols, introducing not only the multiplication sign as now used universally, but also the proportion sign (double colon ::). The book became popular around 15 years later, as mathematics took a greater role in higher education. Wallis wrote the introduction to his 1652 edition, and used it to publicise his skill as cryptographer; in another, Oughtred promoted the talents of Wren.

Other works were a treatise on navigation entitled Circles of Proportion, in 1632, and a book on trigonometry and dialling, and his Opuscula Mathematica, published posthumously in 1676. He invented a universal equinoctial ring dial of two rings.