The Merced River (pronounced mer-SED), in the central part of the U.S. state of California, is a 145-mile (233 km)-long tributary of the San Joaquin River flowing from the Sierra Nevada into the Central Valley. It is most well known for its swift and steep course through the southern part of Yosemite National Park, and the world-famous Yosemite Valley. The river's character changes dramatically once it reaches the foothills and the lowlands, becoming a slow-moving waterway meandering through irrigated fields.

When tectonic activity first led to the uplift of the Sierra, the river formed as a steep stream eroding into the range's western flank, carrying sediments that would later help form the floor of the Central Valley. A rich riparian zone around the Merced once supported millions of migratory birds along the Pacific Flyway, and the river had one of the southernmost runs of salmon in North America. Indigenous people, including the Miwok and Paiute tribes, lived along the river for thousands of years, thriving on the bountiful flora and fauna supported by the river and its diverse lower course, aided by fertile soils eroded from the mountains.

Military expeditions sent by the government of Mexico which then held sovereignty over Alta California passed through the Merced River region in the early 19th century. Later settlers were predominantly American, who discovered gold in the Sierra Nevada, leading to the establishment of a railroad along the river, bringing minerals and lumber to towns that had been established on the lower Merced, and helping increase tourism to the future national park area. Conflicts between whites and indigenous prompted wars, resulting in the expulsion of the Ahwahnechee tribe from Yosemite Valley. In the 20th century, the river saw further development that would change its state forever.