The Thacher School is a highly-selective private co-educational day and boarding school in Ojai, California. Founded in 1889 as a boys' school, it began admitting girls in 1977 and is California's oldest co-educational boarding school, as well as the oldest private boarding school west of the Mississippi River. The school educates approximately 250 students in grades 9–12, who come from 21 states and 12 countries.

Originally a ranch, Thacher's 540-acre campus supports formal horse and outdoors programs. School founder Sherman Day Thacher believed in the power of the outdoors to help shape students: "Come West, breathe deep, let these hills be your teachers." In 2019, the school's older buildings were listed on the U.S. National Register of Historic Places. Famous alumni include industrialist Howard Hughes and playwright Thornton Wilder.

In 1887, Sherman Day Thacher moved to the Casa de Piedra (CdeP) orange ranch in Nordhoff, California (later renamed to Ojai) with his brother, who was seeking a fresh air cure for his tuberculosis. Thacher's father Thomas was a professor at Yale, and in 1889, Thacher's friend Henry W. Farnam (another Yale professor) asked Thacher to tutor his son for Yale's entrance exams. Thacher agreed to do so, provided that the son move to California for his lessons. According to the Los Angeles Times, Thacher is the oldest private boarding school west of the Mississippi.

At CdeP, Thacher tutored the younger Farnam in both academics and maturity, blending classroom studies with outdoor living and horsemanship. Tuition was $14 a week, "washing excluded." Soon other friends were sending their sons out to California to receive Thacher's instruction, and a school was born. By 1900, Thacher had educated 45 students, and its advertisements counted the presidents of Yale, Harvard, Stanford, and Berkeley as references.

Thacher served as an inspiration (direct or indirect) for various other college-preparatory boarding schools. His Yale roommate Horace Dutton Taft visited CdeP in 1889, and Taft's experience helped crystallize his decision to found Connecticut's Taft School the following year. In addition, Curtis Cate taught English at Thacher in the 1900s before establishing Thacher's athletic rival Cate School in 1910. Due to the school's popularity, the admissions office was oversubscribed.^{[clarification needed]} As a result, Sherman Thacher encouraged Thompson Webb to move from Tennessee to Claremont, California and establish The Webb School of California, which opened in 1922.

Thacher incorporated the school as a nonprofit in 1924, but continued running the school until his death in 1931. He was succeeded by Morgan Barnes, who led the school for five years until Thacher's son Anson (CdeP 1923) was ready to take over. Anson Thacher served as head of school from 1936 to 1949, and was the last member of the Thacher family to lead the school. He continued teaching math at Thacher until 1970.

When founding the school, Sherman Thacher imposed "the unusual requirement that each student must care for a horse," remarking that "there's something about the outside of a horse ... that's good for the inside of a boy." There was also a practical element to the horse program: "students had to ride the five-mile trek into [downtown] Ojai every day just to pick up the mail." As of 2023, the horse program requires students to ride and care for a horse during all athletic seasons their freshman year at Thacher, while students who join at later grades complete the requirement during one athletic season. An annual gymkhana event provides students the opportunity to demonstrate horsemanship in competition. Though Western-style riding is required, the Horse Program offers an English riding elective in grades 10–12.

As part of the Outdoors Program, students are encouraged to take weekend camping trips into the local mountains, in addition to week-long trips each fall and spring that include backpacking, rock climbing, cycling, sailing, horse camping, canyoneering, backcountry skiing and kayaking.^{[citation needed]}