Ethan A. Nadelmann (born March 13, 1957) is the founder of the Drug Policy Alliance, a New York City-based non-profit organization working to end the war on drugs. He is a supporter of the legalization of marijuana in America.

Nadelmann was born in New York City and raised in Westchester, New York in a Jewish family; his father, Ludwig Nadelmann, was a rabbi and a "leading figure in the Jewish Reconstructionist movement." He earned B.A., J.D., and Ph.D. degrees from Harvard University and a M.Sc in international relations from the London School of Economics. Nadelmann began to see the flaws in American drug policy as a college student. His academic interests initially focused on Middle East politics before he devoted himself to the issue of drug policy and the internationalization of crime law enforcement. In 1984-85, while pursuing his Ph.D., Nadelmann "got a security clearance and worked as a consultant to the State Department's Bureau of International Narcotics and Law Enforcement Affairs," then called the Bureau of International Narcotics Matters. Nadelmann's dissertation, based in part on "hundreds of DEA and foreign drug-enforcement officials" in 19 countries, was subsequently published as "Cops Across Borders: The Internationalization of U.S. Criminal Law Enforcement." Reviewing the book in Foreign Affairs, David C. Hendrickson called it a "pioneering and prodigiously researched work." Hendrickson commented on how Nadelmann didn't criticize drug policies in the book, writing, "The work contains little in the way of normative judgments or policy prescriptions. Given Nadelmann's known objections to the war on drugs, this gives the book an odd character, quite as if the Rev. Pat Robertson were to attempt a detached scientific analysis of the production values in Last Tango in Paris." Nadelmann then began to focus on the "harms created by drug prohibition" as he taught politics and public affairs at Princeton University from 1987 to 1994.

While he was at Princeton, Nadelmann lectured and wrote extensively on drug policy, starting with a piece in Foreign Policy in April 1988 called "U. S. Drug Policy: A Bad Export." In the article, Nadelmann argued that U.S. drug policy strained relationships with Latin American countries, and Nadelmann "analyzed legalization as an alternative." The article brought Nadelmann media attention alongside Kurt Schmoke, a Baltimore Mayor who advocated for drug decriminalization. Nadelmann appeared on TV shows including Nightline, where he and Schmoke debated Charles Rangel on drugs, and Larry King Live. Nadelmann then "authored similar articles Science, The Public Interest, and New Republic," which were often quoted in op-ed articles about drug policy. Nadelmann's scholarly work provided the intellectual foundation for the legalization effort, and it was responsible for the cross-pollination of varying views on legalization, from libertarian arguments to ones concerning the fiscal burden of the war on drugs.

Nadelmann formed the Princeton Working Group on the Future of Drug Use and Alternatives to Drug Prohibition. The group included eighteen scholars including Lester Grinspoon, Andrew Weil, and Alexander T. Shulgin. Martin Torgoff wrote in Can't Find my Way Home that "for a brief time, the Princeton Group was the most dynamic de facto drug-reform think tank in the United States."

After Barack Obama won the presidential election, Matt Elrod, the director of the drug policy reform group DrugSense, filed an internet petition for Ethan Nadelmann as the new Drug Czar. Although any hopes in getting Nadelmann appointed were downplayed, "this petition will at least encourage President-elect Obama to think twice about his choice of drug czar." Drug Policy Alliance never lobbied for Nadelmann, but once media reports alleged that James Ramstad (R-MN) would be appointed to the post, the organization urged people to oppose the appointment due to his opposition to medical marijuana and needle exchange among other things. Seattle's police chief Gil Kerlikowske became the next head of the White House Office of National Drug Control Policy (ONDCP).

On September 28, 2012, Nadelmann spoke at the Human Rights Foundation's San Francisco Freedom Forum. He discussed the United States' incarceration rates, which are at 743 people per 100,000 inhabitants, and how America's drug policies are affecting that number.

Nadelmann founded the Lindesmith Center in 1994, a drug policy institute created with the support of George Soros. In Soros: The Life and Times of a Messianic Billionaire, Michael T. Kaufman wrote of Nadelmann and Soros's relationship, which formed after Soros read Nadelmann's Spring 1988 piece in Foreign Policy, "U.S. Drug Policy: A Bad Export" as Soros had also published the piece "After Black Monday" in the same issue:

"Soros was so impressed with the drug policy piece that he contacted its author, Ethan Nadelmann. By 1993, with Soros's financial backing, Nadelmann established the Lindesmith Center, a policy institute named after Alfred E. Lindesmith, a sociologist who in the 1930s and 1940s had opposed harsh policies of drug prohibition in favor of medical treatment of addicts. Nadelmann, both brash and persuasive, identified the center's mission as seeking "harm reduction", which he defined as "an alternative approach to drug policy and treatment that focuses on minimizing the adverse effects of both drug use and drug prohibition."