William Dummer Powell (November 5, 1755 - September 6, 1834) was a Loyalist lawyer, judge and political figure in Upper Canada.

He was the eldest son of John Powell (1716-1794), a prosperous merchant who for thirty years held the contract for provisioning the Royal Navy before fleeing to England after the revolution.

[citation needed] Powell studied in Boston before being sent to England under the care of his maternal relative, Sir Alexander Grant, who sent him to board at Tonbridge School.

Having in four years excelled at nothing other than cricket, he was next sent him to live with a merchant at Rotterdam where he was to learn French and Dutch while gaining a first-hand experience in business.

His formal call to the English bar, delayed because of finances, was finally arranged in 1784 and, later that year, he returned to Boston to attempt to recover his father's property which had been confiscated after the American Revolution.

In 1814, he assisted Chief Justice Thomas Scott by presiding over several of the trials known as the "Bloody Assize" which were held at Ancaster to prosecute those charged with treason during the war.

He upset the province's administration by rejecting many of the charges brought by Lord Selkirk against those who had stirred up trouble for the Red River Colony.