William Fairfield (c. 1769 – February 6, 1816) was a businessman and political figure in Upper Canada.
His father fought with Edward Jessup's loyalist forces and moved with his family to Machiche (Yamachiche) in Quebec in 1779.
The younger Fairfield, in charge of a mill, built his own home in the village of Ernestown (Bath) in 1796.
He was a justice of the peace and became a member of the Legislative Assembly of Upper Canada representing Ontario & Addington in 1799, replacing Christopher Robinson, who had died the previous year.
Together with his brothers Benjamin and Stephen, he had interests in mills, shipbuilding and the sale of goods.