A scholarly English occultist active during the late 18th and early 19th century. Barrett is primarily noted for composing the monumental compendium of information on Western magical tradition and ritual known as The Magus, or The Celestial Intelligencer. This enormous (432-page, encyclopedia-sized) volume is one of the finest compendiums on pre-Golden Dawn hermetic magical practices in existence. The book was published in 1801 and had a profound influence on Victorian occultists, particularly Eliphas Levi, as well as groups such as the Hermetic Order of the Golden Dawn; indeed, such groups drew much of their basic resource material from this book.

Little other information on Barrett's life can be found, at least during the course of an Internet search. There are indications that he plagiarized Heinrich Cornelius Agrippa's earlier (but less complete) grimoire, De Occulta Philosophia (published 1533), as parts of each text are identical, right down to errors in printing. Barrett also reprinted sections of the writings of the 15th-century physician and philosopher known as Paracelsus.