Louis Alexander Slotin ( SLOHT-in; 1 December 1910 – 30 May 1946) was a Canadian physicist and chemist who took part in the Manhattan Project. Born and raised in the North End of Winnipeg, Manitoba, Slotin earned both his Bachelor of Science and Master of Science degrees from the University of Manitoba, before obtaining his doctorate in physical chemistry at King's College London in 1936. Afterwards, he joined the University of Chicago as a research associate to help design a cyclotron.

In 1942, Slotin was invited to participate in the Manhattan Project, and subsequently performed experiments with uranium and plutonium cores to determine their critical mass values. After World War II, he continued his research at Los Alamos National Laboratory in New Mexico. On 21 May 1946, he accidentally triggered a fission reaction, which released a burst of hard radiation. He was rushed to the hospital and died nine days later on 30 May. Slotin had become the second fatal victim of a criticality accident in history, following Harry Daghlian, who had died of a related accident with the same plutonium "demon core" the previous year.

Slotin was hailed as a hero by the United States government for reacting quickly enough to prevent the deaths of his colleagues. However, some physicists argue that Slotin's behavior preceding the accident was reckless and that his death was preventable. The accident and its aftermath have been dramatized in several fictional and non-fictional accounts.