The Ontario New Democratic Party is a social-democratic political party in Ontario, Canada. The party currently forms the Official Opposition in Ontario following the 2018 general election. It is a provincial section of the federal New Democratic Party. It was formed in October 1961 from the Co-operative Commonwealth Federation and the Ontario Federation of Labour (OFL).

The 2018 Ontario general election was held on June 7, 2018, to elect the 124 members of the 42nd Parliament of Ontario. The Progressive Conservative Party of Ontario, led by Doug Ford, won 76 of the 124 seats in the legislature and formed a majority government. The Ontario New Democratic Party, led by Andrea Horwath, formed the Official Opposition. The Ontario Liberal Party, led by incumbent Premier Kathleen Wynne, lost official party status in recording both the worst result in the party's 161-year history and the worst result for any incumbent governing party in Ontario. The Green Party of Ontario won a seat for the first time in their history, while the Trillium Party of Ontario lost its single seat gained by a floor-crossing during the 41st Parliament.