William Allen FRS FLS FGS (29 August 1770 – 30 September 1843) was an English Quaker, scientist and philanthropist who opposed slavery and engaged in schemes of social and penal improvement in early 19th-century England.

Allen was born in 1770, the eldest son in the Quaker family of Job Allen (1734–1800), a silk manufacturer, and his wife Margaret Stafford (died 1830). He was educated at a Quaker school in Rochester, Kent, and then went into his father's business. As a young man in the 1790s, he became interested in science. He attended meetings of scientific societies, including lectures at St. Thomas's Hospital and Guy's Hospital, becoming a member of the Chemical Society of the latter establishment. On Job Allen's death, the family silk firm was taken over by his father's assistant.

Allen had concentrated on his own career in the field of pharmacy, taking over the Plough Court chemical business of Joseph Gurney Bevan who retired in 1795. In 1802, he was elected a Fellow of the Linnean Society and lectured on chemistry at Guy's Hospital. A year later he was made president of the Physical Society at Guy's, and on the advice of Humphry Davy and John Dalton also accepted an invitation from the Royal Institution to become one of its lecturers.

In 1807, Allen's original research (on carbon) enabled him to be successfully proposed for election to Fellowship of the Royal Society, bringing him into contact with those who were publishing much of the original scientific research of the day. This strengthened his ties with the eminent Humphry Davy, and in due course with his long-standing friend Luke Howard, who was likewise elected to Fellowship of the Royal Society, though some years later.

Allen was known in commerce for his pharmaceutical company at 2 Plough Court, trading in 1795 as Mildred & Allen while he was in partnership with Samuel Mildred. It was situated off Lombard Street in London. In 1797, Allen brought in Luke Howard as a partner. A later partner was Daniel Bell Hanbury, father of the botanist Daniel Hanbury. By 1856, the company had become Allen & Hanburys. It was acquired in 1958 by Glaxo Laboratories, who retained the Allen & Hanburys name as a separate marque within the GSK group.

Allen strengthened the company's links with medical institutions, particularly Guy's Hospital, where he was elected to its Physical Society. Using Plough Court for meetings, he also co-founded the Askesian Society. There new ideas for research and experimentation could be discussed with others such as Luke Howard, Joseph Fox, William Hasledine Pepys, William Babington, and the surgeon Astley Cooper. A second laboratory was opened for the development of new chemicals, a few miles away in Plaistow.

In 1841, Jacob Bell pressed for a professional body for pharmacists. Allen co-founded The Pharmaceutical Society, which later became The Royal Pharmaceutical Society. He was its first president.

As a teenager, under the influence of Quaker abolitionists, Allen gave up sugar as a reaction to the Atlantic slave trade and abstained from it until 1834. He became interested in the politics of abolitionism in 1790/1, and in 1792, made a speech on the topic at the Worshipful Company of Coachmaker's Hall. The speech was published as a pamphlet by Martha Gurney, which was influenced by writer William Fox. In 1794, he befriended Thomas Clarkson.