The Union of Concerned Scientists (UCS) is a nonprofit science advocacy organization based in the United States. The UCS membership includes many private citizens in addition to professional scientists. Anne Kapuscinski, Professor of Environmental Studies and Director of the Coastal Science and Policy Program at the University of California, Santa Cruz, chairs the UCS Board of Directors as of 2015^[update], having replaced James J. McCarthy, Professor of Biological Oceanography at Harvard University and past president of the American Association for the Advancement of Science.

The Union of Concerned Scientists was founded in 1969 by faculty and students of the Massachusetts Institute of Technology. The organization's founding document says it was formed to "initiate a critical and continuing examination of governmental policy in areas where science and technology are of actual or potential significance" and to "devise means for turning research applications away from the present emphasis on military technology toward the solution of pressing environmental and social problems." The organization employs scientists, economists, and engineers engaged in environmental and security issues, as well as executive and support staff.

One of the co-founders was physicist and Nobel laureate Henry Kendall, who served for many years as chairman of the board of UCS.^{[citation needed]}

In 1992, Kendall presided over the World Scientists' Warning to Humanity, which called for "fundamental change" to address a range of security and environmental issues. The document was signed by 1700 scientists, including a majority of the Nobel prize winners in the sciences.^{[non-primary source needed]}

In 1997, the UCS presented their "World Scientists Call For Action" petition to world leaders meeting to negotiate the Kyoto Protocol. The declaration asserted, "A broad consensus among the world's climatologists is that there is now 'a discernible human influence on global climate.'" It urged governments to make "legally binding commitments to reduce industrial nations' emissions of heat-trapping gases", and called global warming "one of the most serious threats to the planet and to future generations." The petition was signed by "more than 1,500 of the world's most distinguished senior scientists, including the majority of Nobel laureates in science." When a petition that questioned the consensus was circulated by the Oregon Institute of Science and Medicine, a small, privately-funded institute, was signed by more than 17,000 science graduates, UCS declared it a "deliberate attempt to deceive the scientific community with misinformation."

In August 2008, the UCS purchased billboards at the airports in Denver, Colorado, and Minneapolis–Saint Paul, Minnesota, where the Democratic and Republican presidential conventions were to be held. The two nearly identical billboards showed the downtown areas of each convention city in a cross hairs, with the message that "when only one nuclear bomb could destroy a city" like Minneapolis or Denver, "we don't need 6,000." The name of Senator John McCain or Senator Barack Obama follows, with this admonition: "It's time to get serious about reducing the nuclear threat." The billboards were removed after a complaint from Northwest Airlines, the official airline of the Republican convention. The UCS has accused Northwest, whose headquarters were in Minnesota, of "taking on a new role as censor" and of having acted because it regarded the Minneapolis advertisement as "scary" and "anti-McCain."

In June 2020, a UCS staffer named Ruth Tyson resigned and sent a 17-page open letter expressing her opinions on racial inequality in the organization, saying that ideas of black workers were routinely dismissed or given low priority. After reading the letter, UCS president Ken Kimmell responded by saying the criticism was fair, and that he believed it was reflective of a wider culture of white supremacy in society, vowing to address issues and diversify the UCS workforce. His successor, Johanna Chao Kreilick, was chosen in part for "her track record of integrating racial justice into the work and culture of complex organizations".

In February 2025, noted environmental scientist and policy advocate Dr. Gretchen Goldman became the organization's president.