Northfield Mount Hermon School (NMH) is a co-educational college-preparatory school in Gill, Massachusetts. It educates boarding and day students in grades 9–12, as well as post-graduate students. It is a member of the Eight Schools Association.

In 1879, Northfield, Massachusetts, native Dwight Lyman Moody (1837–99) established the Northfield Seminary for Young Ladies (renamed to the Northfield School for Girls in 1944) in his hometown. Two years later, he established a brother school, the Mount Hermon School for Boys, across the Connecticut River in Gill, Massachusetts. The schools were consolidated into a single non-profit corporation in 1912, but operated separately until 1971.

Moody initially envisioned the schools as a source of terminal education; in the early days, some of the students were in their thirties. The schools offered separate programs of study to accommodate their student body's varying goals. Each offered a college-preparatory course and a technical course. For a while, Mount Hermon also offered courses in agriculture and for future ministers. In the early days, most Mount Hermon students enrolled in the ministerial program, whose curriculum was designed to be sufficiently rigorous that a graduate could "enter the ministry or a related field without further formal education".

An Evangelical preacher, Moody sought "to provide a Christian education for [students] of high purpose and limited means". The schools charged low tuition ($100/year in 1881) compared to other boarding schools and relied heavily on donations from Moody's followers. Through the 1920s, the rule was that "[n]o student was accepted if he could afford the fees of more expensive schools"; as a result, the students were "drawn largely from families at or near the poverty line", and, as late as 1914, a majority of male students at Mount Hermon had previously worked in an occupation or trade. In 1903 two-fifths of Northfield students did not live within commuting distance of a high school. Students would attend, drop out, and return based on the family's economic needs back home. In 1903, the schools reportedly enrolled 1,200 students and received at least four applicants for every vacancy.

On campus, the schools tended to provide a "community life of minimum expenditure". The schools operated a campus farm, and all students (both boys and girls) were required to perform some kind of labor to help fund the school's operations. Today, each student is still required to hold a job on campus, working three hours a week.

In the 1920s and 1930s, the Northfield schools shifted to a more conventional college-preparatory boarding school model. Enrollment remained high; by 1930, the schools' combined enrollment made the institution the largest private secondary school in the United States. Mount Hermon's ministerial curriculum was eliminated, and although a minority of Mount Hermon graduates went on to college during the Moody years, by the 1940s "virtually all [Mount Hermon boys] did so", as did half the girls at Northfield.

During the Great Depression, many Americans proved unable to pay even the Northfield schools' relatively low tuition fees. As such, the schools began accepting wealthy students in the 1930s. Tuition increased from $324 in 1929 to $2,600 by 1963, quadrupling in real terms. Nonetheless, the schools still educated large numbers of working- and middle-class students; in 1963, the school announced that it would double its financial aid budget, putting 60% of students on scholarship. The cost of providing a college-preparatory education has increased over time, and the school's reliance on wealthy students has increased accordingly. The percentage of scholarship students halved from 1963 to 2015.

The schools' ties to Evangelical Christianity weakened amidst the Fundamentalist-Modernist Controversy, and the schools eventually shifted to "a more liberal brand of Protestantism". Compulsory attendance at most Sunday chapel services was abolished in 1970.