The William S. Hart Union High School District (commonly referred to simply as the Hart District) is a school district in the Santa Clarita Valley of Los Angeles County, California. It serves all of the valley's high school students (grades 9–12) and most of its junior high school students (grades 7–8). As of 2022–23, the total number of enrolled students (in 16 schools) was 22,469. The district's superintendent is Mike Kuhlman. The district is named after William S. Hart, one of the first western film stars, who lived in Newhall and was a local benefactor. The region served by the Hart District is also served by the Sulphur Springs, Saugus, Castaic, and Newhall elementary school districts.

Prior to the establishment of William S. Hart High School in 1945, students in the Santa Clarita Valley continuing after elementary school would attend San Fernando High School in the Pacoima neighborhood of Los Angeles.

Between the 2015 and 2019, student enrolment within the Hart District declined 11.7% from an all-time high of 27,155 students during the 2015–16 academic year to 23,968 students during the 2019–2020 academic year. The COVID-19 pandemic continued this downward trend, with student enrolment declining a further 6.5% to 22,469 students during the 2022–23 academic year. Despite a number of new housing developments being constructed throughout the Santa Clarita Valley, overall student enrolment is still expected to continue declining for the foreseeable future.

Eight of the district's ten high schools are located within the city of Santa Clarita, with Castaic High School and West Ranch High School being located in the unincorporated communities of Castaic and Stevenson Ranch, respectively.

Jereann Bowman High School serves as the district's continuation high school. The Academy of the Canyons and Learning Post Academy offer students alternative education pathways.

Five of the district's six junior high schools are located within the city of Santa Clarita, with Rancho Pico Junior High being located in the Unincorporated community of Stevenson Ranch.

As of the 2022–23 school year, 22,469 students were enrolled in Hart District schools. 44.2% of students were Hispanic or Latino, 33.4% were non-Hispanic white, 7.8% were Asian American, 5.1% were Filipino, 4.6% were multiracial, 4.4% were African American, 0.2% were Pacific Islander, and 0.1% were Indigenous American.

During the 2015–16 school year, an all-time high of 27,155 students were enrolled in Hart District schools. 39.8% were non-Hispanic white, 39.6% of students were Hispanic or Latino, 6.0% were Asian American, 5.0% were African American, 4.7% were multiracial, 4.3% were Filipino, 0.2% were Indigenous American, and 0.1% were Pacific Islander.

• Castaic Union School District: serves Castaic, Val Verde, and a small portion of northwestern Valencia.
• Newhall School District: serves Stevenson Ranch, southern Valencia, and almost all of Newhall.
• Saugus Union School District: serves Saugus, northern Valencia, and western Canyon Country.
• Sulphur Springs School District: serves the majority of Canyon Country and part of northeastern Newhall.

The Castaic, Newhall, and Saugus Union school districts have offices in Valencia, while the Sulphur Springs School District's office is in Canyon Country. Castaic Union School District serves grades K–8; the other districts serve grades K–6.

Hart District governing board members are composed of five members, elected to a four-year term, by geographical district. The elections are held on a Tuesday after the first Monday in November of even numbered years.

• Official website
• School zone map