Yaphank () is a hamlet and census-designated place (CDP) in Suffolk County, New York, United States. The population was 5,945 at the time of the 2010 census.
Yaphank is located in the south part of the Town of Brookhaven. It is served by the Longwood Central School District, except for extreme southwestern Yaphank, which is served by the South Country Central School District.
Captain Robert Robinson came to Yaphank and built his Dutch Colonial house with the building dated at 1726. He was then granted permission to dam the Carmans River to build a mill across the street from his house. The construction of this mill in 1739 was considered the founding date of the Hamlet of Yaphank.
In the mid 18th century, a man named John Homan built two mills along the Carmans 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".
In 1843 the Long Island Rail Road built a railroad station in Yaphank (still named Millville at the time), and virtually overnight the small mill town became a major commercial center.