David Robert Peterson PC OOnt ECO KC (born December 28, 1943) is a Canadian lawyer and former politician who served as the 20th premier of Ontario from 1985 to 1990. He was the first Liberal officeholder in 42 years, ending the so-called Tory dynasty.

Peterson was born in Toronto, Ontario, to Clarence Marwin Peterson (1913–2009) and Laura Marie Scott (1913–2015), and has two siblings, former MPP Tim Peterson and former MP Jim Peterson. His parents were both born in Saskatchewan. His father was born to Norwegian immigrant farmers who had previously homesteaded in North Dakota. In the early 1930s, Clarence Peterson joined the newly-formed Co-operative Commonwealth Federation and was present at the conference where it adopted the Regina Manifesto. Looking for work during the Great Depression, he moved to Ontario and in 1936 was living in Toronto, where he found a job as a salesman with Union Carbide. The job eventually took him to London, Ontario, where he worked for the company for several years before establishing a wholesale electronics business, C.M. Peterson Co. Ltd, in 1944. He was elected to city council as an alderman in the early 1950s and joined the Ontario Liberal Party, running as its candidate in the 1955 Ontario general election in London North, losing to future premier John Robarts, and ran again as a federal Liberal candidate in 1963 in London.

Peterson grew up in London and earned his Bachelor of Arts degree from the University of Western Ontario in political science and philosophy and his law degree from the University of Toronto. He was called to the bar in 1969. He was made a Queen's Counsel in 1980 and later was appointed to the Queen's Privy Council for Canada in 1992 by Brian Mulroney.

At the age of twenty-six, he became president of C.M. Peterson Company Limited, a wholesale electronics firm founded by his father. He holds four Honorary degrees including a doctor of laws from the University of Western Ontario and is a knight of the Order of the Legion of Honour of France and a member of the Order of La Pléiade. In 2009, he became a member of the Order of Ontario.

Peterson married actress Shelley Matthews in 1974 and they have since raised three children. He is the younger brother of Jim Peterson (1941–2024), formerly a federal Liberal MP and cabinet minister. Both his sister-in-law Deb Matthews and his brother, Tim Peterson, were elected to the Ontario legislature in the 2003 provincial election while Deb Matthews was re-elected in 2007, 2011 and 2014.

Peterson was elected as the Liberal Member of Provincial Parliament for London Centre in the 1975 provincial election. Less than one year later, he campaigned for the leadership of the party following Robert Nixon's resignation. Despite his inexperience, Peterson nevertheless came within 45 votes of defeating Stuart Smith on the third and final ballot of a delegated convention held on January 25, 1976. Smith presented an image of an articulate intellectual who some delegates said reminded them of Pierre Trudeau while Peterson came across as similar to then Premier Bill Davis. Convention delegates also thought that Peterson, a neophyte MPP at 31 years old, was too young and his convention address which he later characterized as the "worst speech in modern political history" came across as stilted and over rehearsed.

Peterson was re-elected in the provincial elections of 1977 and 1981. He ran again for the Liberal leadership in 1982, after Smith resigned.

The convention was held on February 21, 1982. This time his convention speech was better. Although not very inspiring, it was viewed as 'statesmanlike' and effective. He won on the second ballot defeating the more left-leaning Sheila Copps with 55% of the vote. In his acceptance speech Peterson said that he would move party to the 'vibrant middle, the radical centre', and stressed economic growth as a way to increase support for social services. Observers from the other parties felt he was trying to move the Liberal party more to the right, away from values that Smith promoted.