Alameda County is located in the U.S. state of California. As of the 2010 census , the population was 1,510,271, making it the 7th most populous county in the state and the 20th most populous in the nation. The seat of the county is Oakland. Alameda County is part of the San Francisco Bay Area, covering a significant part of the East Bay Area.

The Spanish word "alameda" means either "a grove of poplars or a tree lined lane," a term originally used to describe the Arroyo de la Alameda. The willow and sycamore trees along the banks of the river reminded the early Spanish explorers of a tree lined path. While a strict translation into English may be "Poplar Grove Creek," the name of the main stream that flows through the county is now simply "Alameda Creek."

California became a state on September 9, 1850, but it was not until March 25, 1853 that the Alameda County bill was approved. The county took its name from Alameda Creek, its mainstream, which was the dividing line between Contra Costa and Santa Clara counties. The creek, lined with trees through the otherwise sparsely [sic] wooded plains, gave it the resemblance of a shady walk or lane, which in Spanish is called the Alameda. When the Mexican colonists discovered this part of the state, they knew the name of the place of the Alameda. The exact population of the county at the time of its creation is not known. The second census of Contra Costa County was carried out in 1862, and its entire population, including the part secured by Alameda County in 1853, was just 2,750. However, William Halley, who wrote his history of county in 1876, estimated the population of Alameda County in 1853 at about three thousand, which, he admits, was a considerable increase over the population of two years previous, when it was credited with 752.  

According to the United States of America. Census Bureau, the county has a total area of 821 square miles (2.130 km2) of which 739 square miles (1.910 km2 ) of land and 82 square miles (210 km2) of water. San Francisco Bay surrounds the county to the west.

The crest of the Berkeley Hills is part of the north-eastern border and enters the middle of the county. The coastal plain is a few miles wide stretch of the bay; and Oakland is the most populated area. The Livermore Valley is situated in the eastern part of the county. Amador Valley lies on the western side of the Livermore Valley and heads west to the Pleasanton Ridge.

The Hayward Fault, the main branch of the San Andreas Fault to the west, passes through the most populous areas of the county of Alameda, while the Calaveras Fault passes through the southeastern portion of the county.