James Alexander Renwick (died 1984) was a Canadian politician.

Renwick was a corporate lawyer with a firm that had strong links to the Liberal Party of Canada. Progressively minded and looking for a vehicle to address what he saw as the problems of society, he attended the Liberal Party's 1960 conference in Kingston which was held while the party was in opposition to explore new ideas the party should embrace. However, Renwick was unimpressed and chose not to join the party. He was impressed by the newly formed New Democratic Party of Canada and joined the democratic socialist party after attending the Ontario New Democratic Party' s 1962 convention. He was the NDP's candidate in a 1964 by-election in the Toronto riding of Riverdale against Ontario Liberal leadership candidate Charles Templeton. Renwick's campaign team, led by future Ontario NDP leader Stephen Lewis, developed an innovative techinique of going door to door three times during the campaign in order to identify the party's likeliest voters and then "pulling the vote" on election day. This system maximised the NDP's vote on election day and increased the overall turnout to over 60%, about 1/3 higher than the usual turnout for a by-election, resulting in an upset victory for Renwick. The system was adopted by the NDP throughout Ontario and was later copied by other parties.

The NDP experienced a major breakthrough in the 1967 Ontario election greatly increasing their representation in the Ontario legislature but many party members, and members of caucus, felt the party could have done better under new leadership. Donald C. MacDonald had led the party and its predecessor, the CCF, since 1953 and was seen as out of touch with a younger, more radical mood of the 1960s. Renwick was persuaded to challenge MacDonald for the party leadership in 1968 but was defeated in the leadership vote. Renwick's challenge, though unsuccessful, opened the way for Stephen Lewis to run for the party's leadership in 1970 after MacDonald was persuaded that he could not withstand another leadership challenge and decided to retire.

Renwick remained an MPP until his death in 1984. He also served as president of the federal NDP in the late 1960s.