William Edmund Newton Sinclair (1873-1947) was a Canadian politician. In the 1917 Canadian election on conscription (see: Conscription Crisis of 1917), Sinclair ran as a Laurier Liberal but was defeated. He later ran and was elected to the Ontario legislature. He ran for the leadership of the party in the 1922 Ontario Liberal leadership convention losing to Wellington Hay. Hay resigned following a disastrous election result and Sinclair served as interim leader of the Ontario Liberal Party from 1923 to 1930. He was interim leader for that length of time (and through the elections of 1926 and 1929) due to the party's state of disorganization and inability to hold a proper leadership convention. The Liberals remained at the 14 seats they had in 1926 and dropped to 13 seats in the 1929 election. Sinclair initially decided to run for the permanent leadership in 1930 but due to lack of support (largely because of the party's failure to make gains during his tenure) he withdrew before balloting began. He remained Leader of the Opposition until the 1934 election since Mitchell Hepburn did not have a seat in the provincial legislature.

In the 1945 Canadian election, Sinclair was elected to the Canadian House of Commons for the Liberal Party of Canada dying in office in 1947.