Castro Valley is a census-designated place (CDP) in Alameda County, California, United States.

Before the arrival of European settlers the area was settled by the Chocheño (also spelled Chochenyo or Chocenyo) subdivision of the Ohlone Native Americans.

The area Castro Valley now occupies was part of the extensive colony of New Spain in what was the province of Alta California.

In the 1870s, Lake Chabot, a reservoir and popular park, was built by Chinese laborers living at Camp Yema-Po.

The economy of Castro Valley consists largely of the provision of goods and services for local residents.

[30] The greatest number of people (6,683) are employed by the health care and social assistance industry, followed by the retail trade industry with 1,073 employees and accommodation and food service with 1,044 employees.

[34] Castro Valley has a one-screen movie theater, the Chabot Cinema.

[35] The Harry Rowell Rodeo Ranch is located in Castro Valley and is managed by the Hayward Area Recreation and Park District.

[36] The first public school in Castro Valley is a designated California Historical Landmark.

The one-room schoolhouse was donated for "educational purposes only," by Josiah Grover Brickell in 1866.

[38] The Adobe Art Gallery is a program operated by the Hayward Area Recreation and Park District promoting the visual arts and uses the Adobe building, built as a Works Progress Administration project in 1936.

[39] Castro Valley is an unincorporated community and thus is governed directly by the County of Alameda.

Castro Valley Sanitary District[40] provides refuse and sewer collection services for the majority of the community, with wastewater processed at the Oro Loma Wastewater Treatment Plant in San Lorenzo.

[10][41] In lieu of a city council, Castro Valley is represented by a seven-member Municipal Advisory Council, which is an advisory body appointed to advise the Alameda County Board of Supervisors on local issues.

[42] According to the 2015–2019 American Community Survey, educational attainment for Castro Valley residents at least 25 years old is 91.5% high school graduate and 44.9% bachelor's degree.

[32] Castro Valley is primarily served by the Castro Valley Unified School District,[43] though portions of it are served by the Hayward Unified School District (South of I-580 and West of Grove Way) and the San Lorenzo Unified School District (westernmost part).

[47] Interstate 580, which approaches from the east, makes a turn northward at Castro Valley.

In addition to being served by those two freeways, Castro Valley is served with public transportation by bus system AC Transit, and rapid transit system BART with a station.

It is a Sutter Health facility, and provides basic emergency medical services for the area.