William Wynn Westcott (November 17, 1848 - July 30, 1925) was a British esotericist, coroner, ceremonial magician, and Freemason. A doctor of medicine by profession, he became active in Freemasonry in 1871, becoming Master of his home Lodge in 1874 and later also of the prestigious Quatuor Coronati research lodge, as well as achieving other Masonic distinctions.
He studied the Kabbalah and by 1880 became active in the Societas Rosicruciana in Anglia before co-founding the Hermetic Order of the Golden Dawn with Samuel Liddell MacGregor Mathers in 1887. By then he was also active in the Theosophical Society. He devised and organized the Golden Dawn's rituals with Mathers and William Robert Woodman, who preceded him as Supreme Magus of the S.R.I.A. and like Westcott was one of the foremost exponents of Hermeticism of the time. In 1896, he abandoned public involvement with the Golden Dawn due to pressure regarding his job as a Crown Coroner, with which it was seen as an unseemly association. He continued to head the S.R.I.A. and later was involved with the Golden Dawn breakaway Stella Matutina.