| William John McCart | |
| Office1: | Ontario MPP |
| Term Start1: | 1908 |
| Term End1: | 1911 |
| Predecessor1: | George Kerr |
| Successor1: | John Colborne Milligan |
| Term Start2: | 1902 |
| Term End2: | 1904 |
| Predecessor2: | John McLaughlin |
| Successor2: | George Kerr |
| Constituency2: | Stormont |
| Party: | Liberal |
| Birth Date: | 7 August 1872 |
| Birth Place: | Port Henry, New York |
| Death Place: | Cornwall, Ontario |
| Occupation: | Merchant |
William John McCart (August 7, 1872 - 1933) was an Ontario merchant and political figure. He represented Stormont in the Legislative Assembly of Ontario from 1902 to 1904 and from 1908 to 1911 as a Liberal member.
He was born in Port Henry, New York, the son of John McCart. He was educated in Brockville, Ontario. He was employed as a clerk with companies in Finch and Maxville, before establishing his own business in Maxville. He later opened a general store in Avonmore. McCart married Wilmena Steele in 1892. He was defeated in the 1905 election but was reelected in 1908. McCart later moved to Toronto where he established a wholesale business dealing in fruit. He drowned at Cornwall in 1933.