Vaclav Smil (Czech: [ˈvaːtslaf ˈsmɪl]; born December 9, 1943) is a Czech-Canadian scientist and policy analyst. He is Distinguished Professor Emeritus in the Faculty of Environment at the University of Manitoba in Winnipeg, Manitoba, Canada. His interdisciplinary research interests encompass energy, environmental, food, population, economic, historical and public policy studies. He has also applied these approaches to energy, food and environmental affairs of China.

Smil was born during World War II in Plzeň, at that time in the German Protectorate of Bohemia and Moravia (present-day Czech Republic). His father was a police officer and his mother a bookkeeper. Growing up in a remote mountain town in the Plzeň Region, Smil cut wood daily to keep the home heated. This provided an early lesson in energy efficiency and density.

Smil completed his undergraduate studies and began his graduate work (culminating in the RNDr., an intermediate graduate degree similar to the Anglo-American Master of Philosophy credential, in 1965) at the Faculty of Natural Sciences of Charles University in Prague, where he took up to 35 hours of lectures a week, 10 months a year, for five years. "They taught me nature, from geology to clouds," Smil said. After graduation he refused to join the Communist party, undermining his job prospects, though he found employment at a regional planning office. He married Eva, who was studying to be a physician. In 1969, following the Soviet invasion of Czechoslovakia and Eva's graduation, the Smils emigrated to the United States, leaving the country months before a Soviet travel ban shut the borders. "That was not a minor sacrifice, you know?" Smil says. Smil then received his Ph.D. in geography from the College of Earth and Mineral Sciences of Pennsylvania State University in 1971.

In 1972, Smil took his first job offer at the University of Manitoba where he remained for decades, until his retirement. He taught introductory environmental science courses among other subjects dealing with energy, atmospheric change, China, population and economic development.

Smil is skeptical that there will be a rapid transition to clean energy, believing it will take much longer than many predict. Smil said "I have never been wrong on these major energy and environmental issues because I have nothing to sell," unlike many energy companies and politicians.

Smil noted in 2018 that coal, oil, and natural gas continue to make up 90% of the primary energy sources used in the world. Although renewable energy technologies have improved over time, the global share of energy produced from fossil fuels since 2000 has increased. Smil emphasizes that replacing the use of fossil carbon in the production of primary iron, cement, ammonia, and plastics is a significant and ongoing challenge in the industrial sector. Together, these industries account for 15% of the world's total fossil fuel consumption. Smil stresses the need for energy prices to reflect their true costs, including greenhouse gas emissions, and promotes a decrease in the demand for fossil fuels through energy-saving measures.

Smil believes economic growth has to end, that all growth is logistic rather than exponential, and that humans could consume much lower levels of materials and energy.

Included among Smil's admirers is Microsoft co-founder Bill Gates, who has said: "I wait for new Smil books the way some people wait for the next Star Wars movie." "He's a slayer of bullshit," says David Keith, an energy and climate scientist at Harvard University.