Holtsville is a hamlet and census-designated place (CDP) in Suffolk County, New York, United States. The population was 19,714 at the 2010 census.
Holtsville is mainly in the Town of Brookhaven, while the southwest portion is in the Town of Islip. An IRS Processing Center is located in Holtsville.
The hamlet known today as Holtsville included only a few farmhouses in the late 18th century. In 1843, the Long Island Rail Road opened its Waverly Station. Maps from that period label the area as Waverly, and a stagecoach line ran north–south along present day Waverly Avenue. As another post office named Waverly already existed in Upstate New York, the name of the hamlet was changed to Holtsville in 1860, in honor of U.S. Postmaster General Joseph Holt. As of 1874, Holtsville consisted of 15 houses, a school, and a general store. The train station retained the name "Waverly" for some time, but was eventually also changed to Holtsville, probably in the 1890s, after farmers complained about their shipments going upstate by mistake. In 1916, the Suffolk County Tuberculosis Sanatorium opened on land that was considered Holtsville at the time, but is now part of the hamlet of Selden.