The Ontario New Democratic Party (ONDP or NDP; French: Nouveau Parti démocratique de l'Ontario), is a social democratic political party in Ontario, Canada. It is a provincial section of the federal New Democratic Party. It was formed in October 1961 from the Ontario section of the Co-operative Commonwealth Federation (Ontario CCF) and the Ontario Federation of Labour (OFL).

For many years, the ONDP was the most successful provincial NDP branch outside the national party's western heartland. It had its first breakthrough under its first leader, Donald C. MacDonald in the 1967 provincial election, when the party elected 20 Members of Provincial Parliament (MPPs) to the Ontario Legislative Assembly. After the 1970 leadership convention, Stephen Lewis became leader, and guided the party to Official Opposition status in 1975, the first time since the Ontario CCF did it twice in the 1940s. After the party's disppointing performance in the 1977 provincial election, that included losing second party status, Lewis stepped down and Michael Cassidy was elected leader in 1978. Cassidy lead the party through one campaign, the 1981 election. The party did poorly again, and Cassidy resigned.

In 1982, Bob Rae was elected leader. Under his leadership, in 1985, the party held the balance-of-power with the signing of an accord with the newly elected Liberal minority government. After the 1987 Ontario general election, the ONDP became the Official Opposition again. The 1990 Ontario general election surprisingly produced the ONDP's breakthrough first government in 1990 (when the election was called it looked like the Liberals would win a second majority government). The victory produced the first NDP provincial government east of Manitoba. However, it took power just when Canada's economy was in a recession, and was defeated in 1995. Rae stepped down as leader in 1996.

Howard Hampton was elected leader in at the 1996 Hamilton convention, and lead the party through three elections. Hampton's period as leader saw the ONDP lose official party status twice: after the 1999 and 2003 elections. He was able to regain party status the first time after the governing Progressive Conservatives revised party status requirements in accordance with that election's reduction in the number of seats in the legislature, and the second time after winning a string of by-elections in the mid-2000s. The party maintained party status after the 2007 Ontario general election and he stepped down as leader in 2009.

Andrea Horwath replaced him after she was elected leader at the 2009 leadership convention in Hamilton. Under her leadership in the 2011 Ontario general election, the party elected 17 MPPs to the legislature and in the 2014 Ontario general election, the party elected 21 MPPs.