Willowbrook State School was a state-supported institution for children with intellectual disabilities in the Willowbrook neighborhood of Staten Island in New York City, which operated from 1947 until 1987.

The school was designed for 4,000, but by 1965 it had a population of 6,000. At the time, it was the biggest state-run institution for people with mental disabilities in the United States. Living conditions at the school were crowded and filthy, and during its first decade of its operation outbreaks of hepatitis, primarily hepatitis A, were common. Medical researchers initially carried out research on the disease and its effects at the school, which was discontinued after public outcry. The conditions and questionable medical practices and experiments prompted US Senator Robert F. Kennedy to call it a "snake pit". The institution gained national infamy in 1972, when Geraldo Rivera did an exposé on the conditions there. Public outcry led to its closure in 1987, and to federal civil rights legislation protecting people with disabilities. A February 2020 New York Times investigation found that the survivors of Willowbrook continue to be abused in smaller group homes.

A portion of the grounds and some of the buildings were incorporated into the campus of the College of Staten Island, which moved to Willowbrook in the early 1990s. Records and documentation relating to the experiences of the school residents and staff, and the activities of parents and caregivers who brought about the closure of the school are held by the College of Staten Island Library Archives.

In 1938, plans were drawn up to build a facility for people who had an intellectual disability on 375 acres (152 ha) in the Willowbrook section of Staten Island. The residents didn’t want a facility of the sorts in the area because they feared an uptick in crime. Construction was completed in 1942, but instead of opening for its original purpose, it was converted into a United States Army hospital, and named Halloran General Hospital, after the late Colonel Paul Stacey Halloran. After World War II, proposals were introduced to turn the site over to the Veterans Administration, but in October 1947, the New York State Department of Mental Hygiene opened its facility there. Falling back on previous plans, they started admitting children and adults to the "school". The institution was named Willowbrook State School.

Throughout the first decade of its operation, outbreaks of hepatitis, primarily hepatitis A, were common at the school. This led to controversial medical studies being carried out there by medical researchers Saul Krugman of New York University and Robert W. McCollum of Yale University. Between 1956 and 1971, Krugman and McCollum monitored subjects to gauge the effects of gamma globulin in combating hepatitis. One result of the research was a better understanding of the differences between serum hepatitis, which is spread by blood transfusions, and infectious hepatitis, which is spread directly from person to person and is the more common form. A public outcry forced the research project and medical studies to be discontinued.

Paul A. Offit described Krugman's studies as follows:

In an effort to control outbreaks of hepatitis, the medical staff at Willowbrook consulted Saul Krugman.... Krugman found that hepatitis developed in 90 percent of children admitted to Willowbrook soon after their arrival. Although it was known that hepatitis was caused by a virus, it wasn't known how hepatitis virus spread, whether it could be prevented, or how many types of viruses caused the disease. Krugman used the children of Willowbrook to answer those questions. One of his studies involved feeding live hepatitis virus from others stool samples to sixty healthy children. Krugman watched as their skin and eyes turned yellow and their livers got bigger. He watched them vomit and refuse to eat. All the children fed hepatitis virus became ill, some severely. Krugman reasoned that it was justifiable to inoculate retarded children at Willowbrook with hepatitis virus because most of them would get hepatitis anyway. But by purposefully giving the children hepatitis, Krugman increased that chance to 100 percent.

According to vaccinologist Maurice Hilleman, "They [the Willowbrook studies] were the most unethical medical experiments ever performed on children in the United States." Historian David Rothman notes that, "The research was even included in Henry Beecher's 1966 New England Journal of Medicine listing of 'ethically dubious' experiments." Bioethicist Art Caplan has stated that, "The Willowbrook studies were a turning point in how we thought about medical experiments on retarded children... Children inoculated with hepatitis virus had no chance to benefit from the procedure—only the chance to be harmed."