what county is holtsville ny
Holtsville, NY. Online Offers. Holtsville is a hamlet (and census-designated place) in Suffolk County, New York, United States. The population was 17,006 at the 2000 census. Holtsville is the location of an IRS Processing Center (which has the lowest assigned numerical ZIP Code, 00501) and is in the Town of Brookhaven and the Town of Islip.