Ultrafiltration (UF)  is a type of membrane filtration in which hydrostatic pressure forces a liquid against a semi-permeable membrane to physically remove suspended particles from water based on particle size and the pore size .

A semipermeable membrane is a thin layer of material capable of separating substances when a driving force is applied across the membrane.

Ultrafiltration uses hollow fibres of membrane material and the feed water flows either inside the shell, or in the lumen of the fibres.

Suspended solids and solutes of high molecular weight are retained, while water and low molecular weight solutes pass through the membrane.

UF membranes are available in a range of "tightness" or pore sizes and are typically specified based on the molecular weight cutoff, expressed in Daltons, to describe size of molecules they will allow to pass through the membrane.

Ultrafiltration is not fundamentally different from reverse osmosis, except in terms of the size of the molecules it retains.

Ultrafiltration can be used as pre-treatment for reverse osmosis systems or as final filtration stage for deionized water.