Memorial Sloan Kettering Cancer Center (MSK or MSKCC) is a cancer treatment and research institution in Manhattan in New York City. MSKCC is one of 72 National Cancer Institute–designated Comprehensive Cancer Centers. Its main campus is located at 1275 York Avenue between 67th and 68th Streets in Manhattan.

It was formed in 1980 from the merger of the Memorial Hospital for the Treatment of Cancer and Allied Diseases, founded in 1884, and the adjacent Sloan-Kettering Institute for Cancer Research, founded in 1945. The two medical entities had formally coordinated their operations since 1960.

The hospital was founded in its original building on the Upper West Side of Manhattan in 1884 as New York Cancer Hospital by a group that included John Jacob Astor III and his wife Charlotte. The hospital appointed as an attending surgeon William B. Coley, who pioneered an early form of immunotherapy to eradicate tumors. Rose Hawthorne, daughter of author Nathaniel Hawthorne, trained there in the summer of 1896 before founding her own order, Dominican Sisters of Hawthorne. In 1899, the hospital was renamed General Memorial Hospital for the Treatment of Cancer and Allied Diseases. In 1902, Arabella Huntington made a US$100,000 (equivalent to $3.6 million in 2024) bequest in memory of her late husband Collis Potter Huntington to establish the first cancer research fund in the country, the Huntington Fund for Cancer Research.

Around 1910, James Ewing, a professor at Cornell University's medical college, established a collaboration with Memorial Hospital with the help and funding of industrialist and philanthropist James Douglas, who gave $100,000 (equivalent to $3.4 million in 2024) to endow twenty beds for clinical research, equipment for working with radium, and a clinical laboratory for that purpose. Douglas' enthusiasm and funding for development of radiation therapy for cancer inspired Ewing to become one of the pioneers in developing this treatment. Ewing soon took over effective leadership of clinical and laboratory research at Memorial. In 1916 the hospital was renamed again, dropping "General" to become known as Memorial Hospital for the Treatment of Cancer and Allied Diseases.

The first fellowship training program in the U.S. was created at Memorial in 1927, funded by the Rockefellers. In 1931 the then-most-powerful 900k-volt X-ray tube was put into use in radiation-based cancer treatment at Memorial; the tube had been built by General Electric over several years. In 1931 Ewing was formally appointed president of the hospital, a role he had effectively played until then, and was featured on the cover of Time magazine as "Cancer Man Ewing"; the accompanying article described his role as one of the most important cancer doctors of his era. He worked at the Memorial until his retirement, in 1939. Under his leadership, Memorial became a model for other cancer centers in the United States, combining patient care with clinical and laboratory research, and it was said of him that "the relationship of Ewing to the Memorial Hospital can best be expressed in the words of Emerson, 'Every institution is but the lengthening shadow of some man.' Dr. Ewing is the Memorial Hospital".

In 1934, John D. Rockefeller Jr. donated land on York Avenue for a new location. Two years later, he granted Memorial Hospital for the Treatment of Cancer and Allied Diseases (Memorial Hospital) $3.0 million (equivalent to $68 million in 2024) and the hospital began their move across town. Memorial Hospital officially reopened at the new location in 1939. In 1945, the chairman of General Motors, Alfred P. Sloan, donated $4.0 million (equivalent to $69.9 million in 2024) to create the Sloan-Kettering Institute for Cancer Research through his Sloan Foundation, and Charles F. Kettering, GM's vice president and director of research, personally agreed to oversee the organization of a cancer research program based on industrial techniques. The originally independent research institute was built adjacent to Memorial Hospital.

In 1948, Cornelius P. Rhoads became the director of Memorial. Rhoads had run chemical weapons programs for the United States Army in World War II, and had been involved in the work that led to the discovery that nitrogen mustards could potentially be used as cancer drugs. He fostered a collaboration between Joseph H. Burchenal, a clinician at Memorial and Gertrude B. Elion and George H. Hitchings at Burroughs Wellcome, who discovered 6 MP; the collaboration led to the development and eventual wide use of this cancer drug.

From the mid-1950s to the mid-1960s Chester M. Southam conducted pioneering clinical research on virotherapy and cancer immunotherapy at MSK; however he conducted his research on people without their informed consent. He did this to patients under his care or others' care, and to prisoners. In 1963 some doctors objected to the lack of consent in his experiments and reported him to the Regents of the University of the State of New York which found him guilty of fraud, deceit, and unprofessional conduct, and in the end, he was placed on probation for a year. Southam's research experiments and the case at the Regents were covered in The New York Times.