Willem Boreel (1591–1668) was a Dutch diplomat.
He was the son of Jacob Boreel (died 1636), burgomaster of Bergen-op-Zoom. Adam Boreel and the jurist Abraham Boreel were brothers; Johan Boreel was a half-brother. He was knighted by James I of England in 1618. As a lawyer, Boreel worked for the Dutch East India Company, and was part of a mission to resolve the Dutch and British commercial rivalry in the East Indies by a treaty.
Boreel became Baron of Vreendijk and Vreenhove. From 1627 to 1649 he was Pensionary of Amsterdam. During that period he travelled to England, with Johan van Reede van Renswouden, in an attempt to mediate in the First English Civil War. Then from 1650 for the rest of his life he served as Ambassador from the Dutch Republic to France.
In 1655 Boreel intervened almost decisively in the controversy over the invention of the telescope. He gave credit to Zacharias Janssen, over others who were more celebrated. This conclusion was adopted by Pierre Borel in his 1656 book on the subject.
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