The Sierra Club is a prominent American environmental organization founded on May 28, 1892, in San Francisco, California, by naturalist John Muir and a group of preservationists. It is one of the oldest and largest grassroots environmental organizations in the United States. Emerging from the progressive movement as one of the world's first large-scale environmental preservation groups, the organization is dedicated to conservation, wilderness protection, advocating for policies that promote environmental sustainability and climate action, while challenging policies harmful to the environment. Its original mission focused on exploring, enjoying, and rendering accessible the mountain regions of the Pacific Coast, particularly the Sierra Nevada, while publishing information and enlisting public and governmental support to preserve forests and natural resources. Early successes included defeating proposals to reduce Yosemite National Park's boundaries in 1892 and supporting the establishment of the Sierra Forest Reserve in 1893. The organization quickly evolved to include political action, such as lobbying for national parks like the Grand Canyon and opposing developments threatening wilderness areas.

Over the 20th century, the Sierra Club expanded its scope and influence nationally, opening chapters in all 50 U.S. states, Washington D.C., and Puerto Rico, with headquarters in Oakland, California. It played pivotal roles in major conservation victories, including preventing dam construction in Dinosaur National Monument (1956), establishing Point Reyes and Padre Island National Seashores (1962), and advocating for key legislation such as the Wilderness Act (1964), the creation of the Environmental Protection Agency (1970), and the Clean Air Act (1977). The club established a charitable foundation in 1960 and a legal defense fund in 1971 to support its growing litigation and advocacy efforts. It has consistently opposed harmful energy sources like coal, hydropower, and nuclear power while promoting sustainable energy. Membership grew from 350 in 1897 to over 15,000 by 1960, reflecting its broadening appeal.

In the modern era, the Sierra Club continues to lobby local, state, and federal bodies for environmental legislation, engage in lawsuits—such as a 2025 action against the U.S. Department of Transportation for withholding records—and endorse liberal and progressive candidates in elections. It has acknowledged controversies, including the racist legacy of founder John Muir in 2020 amid national racial unrest, and faced internal leadership changes, such as the 2023 firing of executive director Ben Jealous. The organization maintains adversarial relationships with some environmental groups, like the Environmental Defense Fund, over policy differences, and remains active in public education, outings, and campaigns against environmental rollbacks.