County routes in Nassau County, New York are maintained by the Nassau County Department of Public Works. Route numbers were originally posted on unique blue-on-orange pentagonal route markers, which reflected the county's official colors; however, all county route signage was removed in the mid-1970s after the Federal Highway Administration enacted new standards for county route markers in 1973. The county legislature refused to allocate funds to replace the signs with new markers conforming to the federal government's Manual on Uniform Traffic Control Devices, leaving the routes unsigned. The route numbers are still used by the county for internal purposes only. County route designations in Nassau County are of two types: some routes are numbered, while others feature a letter followed by a number for their designations.

Numbered routes

Route numbers, lengths, and termini are derived from the New York State Department of Transportation's county road listing for Nassau County, unless otherwise noted.

Lettered routes

This list of county routes (derived from the New York State Department of Transportation's county road listing for Nassau County) have designations that begin with a letter, followed by a number.