William of Baskerville (Italian: Guglielmo da Baskerville, pronounced [ɡuʎˈʎɛlmo dа ˈbaskervil]) is a fictional Franciscan friar from the 1980 historical mystery novel The Name of the Rose (Il nome della rosa) by Umberto Eco.

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The Name of the Rose is itself a recounting of events as experienced by Adso of Melk, a Benedictine novice (a Franciscan one in the 1986 film adaptation) who travelled under William's protection.

Years before the main events of the novel, as an inquisitor, Brother William presided at some trials in England and Italy, where he distinguished himself by his perspicacity along with great humility. In numerous cases Willam decided the accused was innocent. In one of his most consequential cases, William refused to condemn a man on charges of heresy, despite the demands of the inquisitor Bernardo Gui. The accusations of heresy stemmed from the man's translation of a Greek book that contradicted the scriptures. Despite his appeals to the Pope, William was imprisoned and tortured until he recanted, in turn leading to the translator's death by burning at the stake. Though he departed from his role as an inquisitor, his torture and the death of the accused remain fresh in his mind.

In 1327, William and Adso travel to a Benedictine monastery in Northern Italy to attend a theological disputation between the Franciscans and Papal emissaries on the poverty of Christ. This abbey is being used as an embassy between Pope John XXII, and the Friars Minor, who are suspected of heresy.

The abbey boasts a famed scriptorium where scribes copy, translate or illuminate books. After a string of unexpected deaths the abbot seeks help from William, who is renowned for his deductive powers, to investigate the deaths. William is reluctantly drawn in by the intellectual challenge and his desire to disprove fears of a demonic culprit. William also worries the abbot will summon officials of the Inquisition should the mystery remain unsolved.

Following the events of The Name of the Rose, Adso and William part ways, with Adso relating the tale before his death. We are informed near the end of the book that William had died earlier during a Plague in Europe.

The fictional friar, William of Baskerville, alludes both to the fictional sleuth Sherlock Holmes and to William of Ockham. The name itself is derived from William of Ockham and Sir Arthur Conan Doyle's book The Hound of the Baskervilles. Another view is that Eco has created Brother William as a combination of Roger Bacon, William of Ockham and Sherlock Holmes.^[1] (William himself notes that Bacon was a mentor of his and cites his ideas several times in the course of the book.)

William of Ockham, who lived during the time of the novel, first put forward the principle known as "Ockham's Razor", which is often summarised as the dictum that one should always accept as most likely the simplest explanation that accounts for all the facts. William applies this dictum in a manner analogous to the way Sherlock Holmes applies his similar dictum, that when one has eliminated the impossible, whatever remains — however improbable — must be the truth.

In the book, The Name of the Rose, Umberto Eco's description of Brother William of Baskerville has some similarities to Arthur Conan Doyle's description of Sherlock Holmes in A Study in Scarlet (1887).

Sean Connery portrayed William of Baskerville in the 1986 film adaptation The Name of the Rose. John Turturro portrayed William of Baskerville in the 2019 miniseries The Name of the Rose.

• Sean Connery, here depicted in November 1983, portrayed William of Baskerville in the 1986 film adaptation The Name of the Rose.
• John Turturro portrayed William of Baskerville in the 2019 miniseries The Name of the Rose.