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

On January 3, 1973, Ellsberg was charged under the Espionage Act of 1917 along with other charges of theft and conspiracy, carrying a total maximum sentence of 115 years. Due to governmental misconduct and illegal evidence-gathering, and the defense by Leonard Boudin and Harvard Law School professor Charles Nesson, Judge William Matthew Byrne Jr. dismissed all charges against Ellsberg on May 11, 1973.

Ellsberg was awarded the Right Livelihood Award in 2006. He is also known for having formulated an important example in decision theory, the Ellsberg paradox, his extensive studies on nuclear weapons and nuclear policy, and for having voiced support for WikiLeaks, Chelsea Manning, and Edward Snowden.

Ellsberg was awarded the 2018 Olof Palme Prize for his "profound humanism and exceptional moral courage."

By 1969, Ellsberg began attending anti-war events while still remaining in his position at RAND. In April 1968, Ellsberg attended a Princeton conference on “Revolution in a Changing World,” where he met Gandhian peace activist Janaki Tschannerl from India, who had a profound influence on him, and Eqbal Ahmed, a Pakistani fellow at the Adlai Stevenson Institute later to be indicted with Rev. Philip Berrigan for anti-war activism. Ellsberg particularly recalls Tschannerl saying “In my world, there are no enemies”, and that “she gave me a vision, as a Gandhian, of a different way of living and resistance, of exercising power nonviolently."[10]

He experienced an epiphany attending a War Resisters League conference at Haverford College in August 1969, listening to a speech given by a draft resister named Randy Kehler, who said he was "very excited" that he would soon be able to join his friends in prison.

Decades later, reflecting on Kehler's decision, Ellsberg said: Randy Kehler never thought his going to prison would end the war. If I hadn't met Randy Kehler it wouldn't have occurred to me to copy [the Pentagon Papers]. His actions spoke to me as no mere words would have done. He put the right question in my mind at the right time.

Since the end of the Vietnam War, Ellsberg has continued his political activism, giving lecture tours and speaking out about current events. Reflecting on his time in government, Ellsberg has said the following, based on his extensive access to classified material:

The public is lied to every day by the President, by his spokespeople, by his officers. If you can't handle the thought that the President lies to the public for all kinds of reasons, you couldn't stay in the government at that level, or you're made aware of it, a week. ... The fact is Presidents rarely say the whole truth—essentially, never say the whole truth—of what they expect and what they're doing and what they believe and why they're doing it and rarely refrain from lying, actually, about these matters.

In a speech on March 30, 2008 in San Francisco's Unitarian Universalist church, Ellsberg observed that House Speaker Nancy Pelosi does not have the authority to declare impeachment "off the table," as she had done with respect to George W. Bush. The oath of office taken by members of congress requires them to "defend the Constitution of the United States against all enemies, foreign and domestic". He also pointed out that under Article VI of the U.S. Constitution, treaties, including the United Nations Charter and international labor rights accords that the United States has signed, become the supreme law of the land that neither the states, the president, nor the congress have the power to break. For example, if the Congress votes to authorize an unprovoked attack on a sovereign nation, that authorization wouldn't make the attack legal. A president citing the authorization as just cause could be prosecuted in the International Criminal Court for war crimes.

On December 9, 2010, Ellsberg appeared on The Colbert Report where he commented that the existence of WikiLeaks helps to build a better government.

On March 21, 2011, Ellsberg, along with 35 other demonstrators, was arrested during a demonstration outside the Marine Corps Base Quantico, in protest of Manning's current detention at Marine Corps Brig, Quantico.

On June 10, 2013, Ellsberg published an editorial in The Guardian newspaper praising the actions of former Booz Allen worker Edward Snowden in revealing top-secret surveillance programs of the NSA. Ellsberg believes that the United States has fallen into an "abyss" of total tyranny, but said that because of Snowden's revelations, "I see the unexpected possibility of a way up and out of the abyss."

In 2012, Ellsberg became one of the co-founders of the Freedom of the Press Foundation. Ellsberg is a founding member of the Veteran Intelligence Professionals for Sanity.