Holtsville is a hamlet and census-designated place (CDP) in Suffolk County, on Long Island, in New York, United States.

An IRS Processing Center is located in Holtsville, along with NYPA's Richard M. Flynn Power Plant.

[3] 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.

[4][5] 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.

[3] The rail era in Holtsville ended in 1998, when a number of LIRR stations closed due to low ridership.

[8] According to the United States Census Bureau, the CDP has a total area of 7.1 square miles (18.4 km2), all land.

[11] Some of the amenities at the park include: a triple pool complex; exercise-trail fitness course; nature preserve and ecology center, featuring buffalo, bobcats, eagles, ecology exhibits and tours, free compost, greenhouses and a picnic area.