William Carstares (also Carstaires) (February 11, 1649 - December 28, 1715), Scottish clergyman, was born at Cathcart, near Glasgow.

He was the son of the Rev. John Carstares , a member of the extreme Covenanting party of Protestors. He was educated at the university of Edinburgh, and then passed over to Utrecht, where he commenced his lifelong friendship with the prince of Orange, and began to take an active part in the politics of his country. The government disliked Carstares for several reasons. He was the intimate of William; he had been the bearer of messages between the disaffected in Scotland and Holland; and he was believed to be concerned with Sir James Steuart (1635-1715) in the authorship of a pamphlet An Account of Scotland's Grievances by reason of the D. of Lauderdale's Ministrie, humbly tendered to his Sacred Majesty.

Accordingly, on his return to England, at the close of 1674, he was committed to the Tower; the following year he was transferred to Edinburgh Castle, and it was not till August 1679 that he was released. After this he visited Ireland, and then became pastor to a Nonconformist congregation at Cheshunt. During 1682 he was in Holland, but in the following year he was again in London, and was implicated in the Rye House Plot. On its discovery he was examined before the Scottish Council ; though the torture of the thumb-screw was applied, he refused to utter a word till he was assured that his admissions would not be used in evidence, and in the disclosures he then made he displayed great discretion.

On his return to Holland he was rewarded by William's still warmer friendship, and the post of court chaplain; and after the Revolution he continued to hold this office, under the title of royal chaplain for Scotland. He was the confidential adviser of the king, especially with regard to Scottish affairs, and rendered important service in promoting the Revolution Settlement. On the accession of Anne, Carstares retained his post as royal chaplain, but resided in Edinburgh, having been elected principal of the university. He was also minister of Greyfriars , and afterwards of St Giles, and was four times chosen moderator of the general assembly.

He took an important part in promoting the Union, and was consulted by Harley and other leading Englishmen concerning it. During Anne's reign, the chief object of his policy was to frustrate the measures which were planned by Lord Oxford to strengthen the Episcopalian Jacobites especially a bill for extending the privileges of the Episcopalians and the bill for replacing in the hands of the old patrons the right of patronage, which by the Revolution Settlement had been vested in the elders and the Protestant heritors.

On the accession of George I, Carstares was appointed, with five others, to welcome the new dynasty in the name of the Scottish Church. He was received graciously, and the office of royal chaplain was again conferred upon him. A few months after he was struck with apoplexy, and died on the 28th of December 1715.