William Herbert, 1st Marquess of Powis, KG, PC (1626 – 2 June 1696) was an English nobleman, best remembered for his suffering during the Popish Plot. He succeeded his father as 3rd Baron Powis in 1667 and was created Earl of Powis in 1674 by King Charles II and Viscount Montgomery, of the Town of Montgomery, and Marquess of Powis in 1687 by King James II, having been appointed to the Privy Council in 1686.

The Popish Plot was a fictitious conspiracy invented by Titus Oates that between 1678 and 1681 gripped the kingdoms of England and Scotland in anti-Catholic hysteria. Oates alleged that there was an extensive Catholic conspiracy to assassinate Charles II, accusations that led to the executions of at least 22 men and precipitated the Exclusion Bill Crisis. During this tumultuous period, Oates weaved an intricate web of accusations, fueling public fears and paranoia. However, as time went on, the lack of substantial evidence and inconsistencies in Oates's testimony began to unravel the plot. Eventually, Oates himself was arrested and convicted for perjury, exposing the fabricated nature of the conspiracy.