Yaphank is a census-designated place (CDP) in Suffolk County, New York, United States. The population was 5,025 at the 2000 census.
Yaphank is a community in the south part of the Town of Brookhaven. It is served by the Longwood Central School District, except for extreme southwestern Yaphank, where it is served by the South Country Central School District.
In the mid 18th century, a man named John Homan built two mills along Carmen's River, which runs directly through the center of the town. These two mills inspired the first name for the town: Millville. The translator author Mary Louise Booth was born in Millville in 1831. In 1846 a post office was opened in the town, but because there were thirteen other towns named "Millville" in New York State at the time, the town was renamed "Yaphank", from the local Native American word "Yamphanke", meaning "bank of a river".