Stuart L. Smith (born 1938) is a politician, psychiatrist, academic and public servant in Ontario, Canada. He graduated in medicine from McGill University, and taught for several years at McMaster Medical School in Hamilton, Ontario.

In 1965, Smith ran for the Liberal Party nomination in the riding of Mount Royal, and lost to Pierre Trudeau, who went on to become Liberal Party leader and Prime Minister of Canada.

Dr. Smith was elected to the Legislative Assembly of Ontario in the 1975 provincial election. In January 1976, Smith was elected leader of the Ontario Liberal Party.

In the election of 1977, even though the Liberals lost one seat in the Legislature, they became the Official Opposition. The Ontario New Democratic Party lost more seats in the election, and fell to third place in the Legislature standings. Smith therefore became Leader of the Opposition to the Progressive Conservative minority government of William Davis.

As a politician, Smith had a reputation as an intelligent but dry and slightly aloof personality in a province that had grown accustomed to avuncular leaders. His criticism of government spending by the government of Premier Davis, and his pessimism about their election promises, prompted the Tories to nickname him Dr. No - a label which stuck. The Liberals failed to make any progress in the 1981 election, returning again with 34 seats while the Tories regained their majority government.

Smith resigned as leader and left the legislature in January 1982 upon the election of his successor as Liberal leader, David Peterson.

From 1982 to 1987, Dr. Smith was chairman of the Science Council of Canada , a federal government body. In 1988, he founded RockCliffe Research and Technology Inc., a firm which introduced public-private partnerships into government laboratories.

From 1995 to 2002, he was chair of the National Round Table on the Environment and the Economy. From 1994 until June of 1997, Dr. Smith was founding president of Philip Utilities Management Corporation, which became Canada's largest private company in the management of public water and wastewater systems.