East Islip is a hamlet and CDP in the Town of Islip, Suffolk County, New York, United States. At the time of the 2010 census, the CDP had a population of 14,475.
Originally referred to as "East of Islip", the name was acquired in 1890 from the estate of William Nicoll, an English aristocrat who purchased the surrounding land in 1683 to erect a family residence. Nicoll's purchase comprised 51,000 acres (210 km2) from the Secatogue Indians, reaching as far as Bayport to the east, Babylon to the west and Ronkonkoma to the north. His mansion, Islip Grange, named after his family's ancestral estate at Northampshire, England, stood near the shoreline of what is now Heckscher State Park.
East Islip, as well as many hamlets along Long Island's south shore, was once an enclave for some of the nation's wealthiest families. Its estates at one time included the Hollins, Gulden, and Knapp estates, among others. An original estate mansion, Brookwood Hall, has passed from its last private owners, the Thorne family (originally of Great Neck during the Dutch colonial era) and now serves as a municipal building for the Town of Islip. The Sullivan estate became the home of the Hewlett School, a private boarding school which closed shortly after the turn of the century. Some estate and early farmlands were donated to the Roman Catholic church and make up the current grounds of St Mary's of East Islip, which includes a private elementary and middle school, in addition to church and other parish buildings. The original Westbrook farm on the boundary between East Islip and Oakdale, near the Bayard Cutting Arboretum, has ceased operations, and its fields are now the home of East Islip Soccer, near the fields set aside for the Little League of the Islips.