Daniel Ellsberg (April 7, 1931 – June 16, 2023) was an American political activist, economist, and United States military analyst. While employed by the RAND Corporation, he precipitated a national political controversy in 1971 when he released the Pentagon Papers, a top-secret Pentagon study of U.S. government decision-making in relation to the Vietnam War, to The New York Times, The Washington Post, and other newspapers.

In January 1973, Ellsberg was charged under the Espionage Act of 1917 along with other charges of theft and conspiracy, carrying a maximum sentence of 115 years. Because of governmental misconduct and illegal evidence-gathering (committed by the same people who later would be involved in the Watergate scandal), and his defense by Leonard Boudin and Harvard Law School professor Charles Nesson, Judge William Matthew Byrne Jr. dismissed all charges against Ellsberg in May 1973.

Ellsberg was awarded the Right Livelihood Award in 2006. He was also known for having formulated an important example in decision theory, the Ellsberg paradox; for his extensive studies on nuclear weapons and nuclear policy; and for voicing support for WikiLeaks, Chelsea Manning, and Edward Snowden. Ellsberg was awarded the 2018 Olof Palme Prize for his "profound humanism and exceptional moral courage". He was a founding member of Veteran Intelligence Professionals for Sanity.

Ellsberg was born in Chicago, Illinois, on April 7, 1931, the son of Harry and Adele (Charsky) Ellsberg. His parents were Ashkenazi Jews who had converted to Christian Science, and he was raised as a Christian Scientist. In 2008, Ellsberg told a journalist that his parents considered the family Jewish, "but not in religion."

Ellsberg grew up in Detroit and attended the Cranbrook School in nearby Bloomfield Hills. His mother wanted him to be a concert pianist, but he stopped playing in July 1948, two years after both his mother and sister were killed when his father fell asleep at the wheel and crashed the family car into a bridge abutment.

Ellsberg entered Harvard College on a scholarship, graduating summa cum laude with an A.B. in economics in 1952. He studied at King's College, Cambridge, for a year through funding from the Woodrow Wilson National Fellowship Foundation, initially for a diploma in economics and then changed his credits toward a PhD in the subject, before returning to Harvard. In 1954, he enlisted in the United States Marine Corps and earned a commission. He served as a platoon leader and company commander in the 2nd Marine Division, and was discharged in 1957 as a first lieutenant. Ellsberg returned to Harvard as a Junior Fellow in the Society of Fellows for two years.

Ellsberg began working as a strategic analyst at the RAND Corporation for the summer of 1958 and then permanently in 1959. He concentrated on nuclear strategy, working with leading strategists such as Herman Kahn and challenging the existing plans of the United States National Security Council and Strategic Air Command.

Ellsberg completed a PhD in economics from Harvard in 1962. His dissertation on decision theory was based on a set of thought experiments that showed that decisions under conditions of uncertainty or ambiguity generally may not be consistent with well-defined subjective probabilities. Now known as the Ellsberg paradox, it formed the basis of a large literature that has developed since the 1980s, including approaches such as Choquet expected utility and info-gap decision theory.