Nanofiltration (NF) or nanofiltration membrane technology is a relatively recent membrane filtration process used most often with low total dissolved solids water such as surface water and fresh groundwater, with the purpose of softening (polyvalent cation removal) and removal of disinfection by-product precursors such as natural organic matter and synthetic organic matter.

The membranes are key to the performance of NF systems. They are produced in spiral wound, tubular, and hollow fiber formats, from a range of materials, including cellulose derivatives and synthetic polymer.

NF membranes have pore sizes from 1-10 nanometers, smaller than that used in microfiltration and ultrafiltration, but just larger than that in reverse osmosis.