Fort Sill Indian School was an American Indian boarding school near Lawton, Comanche County, Oklahoma, United States. The school opened in 1871, with 24 students in the first year, had 300 students in the 1970s, and closed in 1980 although "Native students and administrators, alumni, and Indian leaders fought tenaciously to keep the school alive when the BIA announced its imminent closure". It was founded by Quakers but became nonsectarian in 1891.

Building 309 of the school is recorded on the National Register of Historic Places, #73001559.

The British Museum holds a collection of 91 photographs taken in the 1990s identified as "Photographs taken for a news story for the Daily Oklahoman on the planned re-opening of the school as a Native American College".

Fort Sill Indian School was founded by Quaker Farmer Lawrie Tatum after he became a federal agency for the Kiowa, Comanche, and Apache tribes of the Fort Sill area in 1869. He relocated the agency one mile west of its original location due to flooding issues and built the Fort Sill Agency School. The first teachers were Josiah and Elizabeth Butler of the Quaker Church of Ohio. The school became non-sectarian, however, after Josiah Butler’s exit in 1891. The school transitioned back to its original campus from 1898-1900 and then became Fort Sill Indian School with its first class being a small number of girls. Local Indigenous peoples resisted enrolling their children at first, but that did not continue for very long.

When the school first opened, the Comanche and Kiowa people refused to enroll their children due to conflict with the United States government. Both the Comanches and Kiowas resisted enrolling their children until 1874 when another federal agent for Indigenous people, Agent Haworth, created a school board that included local Indigenous members. The board included two men from each tribe in the agency and were changed often to ensure communication with as many tribal leaders as possible and spread the influence of men who supported the school. By 1875 twenty Kiowa children were enrolled, making up a third of the school’s entire population. This led to the school’s first problem regarding overenrollment, but it would soon prove to be a regular problem at Fort Sill Indian School.

Like many other Native American boarding schools, daily life at Fort Sill involved militaristic-style discipline including precision marching between campuses and harsh corporal punishment when rules were broken. This was most common in the earlier years of the school’s existence, even though most students at this time were of elementary age. One of the main reasons for punishment was students speaking their native language or practicing their culture in any form. Students were stripped of cultural items and markers on their bodies such as clothing or hair styles when they enrolled in the school to assimilate them into white society, as was the goal of every Native American boarding school.

There were many school programs available to students at Fort Sill, but most of them were focused on vocational skills rather than academics. For example, trade skills and homemaking classes were the most common courses in the 1940s and 50s, including sewing, child care and development, family relations, nutrition, and consumer education. There was also a six-year agriculture program that ran until 1960 when it was replaced with more academic classes. There were also athletic programs and sports teams for the students, but the lack of resources allocated for the school made them rather dangerous to participate in. It was programs like this, however, that helped students at Fort Sill foster positivity even in a hostile environment.

After the first instance of crowding in 1875, over-enrollment became a persistent problem at Fort Sill Indian School. This was partially due to the fact that the school was better equipped and more technologically advanced than other Native American boarding schools in the surrounding areas so children would be sent to Fort Sill after seeing that their local school was not as good. This, however, led to extreme crowding that worsened conditions for all students and staff such as a trachoma outbreak in 1911 that affected nearly 80 percent of the school. Government administrators tried to combat this by expanding and renovating the school, but these additions were never built fast enough and were often poorly executed.