An ion exchange resin or ion exchange polymer is an insoluble matrix normally in the form of small (0.315-1.25 mm diameter) beads, usually white or yellowish, fabricated from an organic polymer substrate. The beads are typically porous, providing a high surface area. The trapping of ions occurs with concomitant releasing of other ions; thus the process is called ion exchanage. There are multiple types of ion exchange resin. Most commercial resins are made of polystyrene sulfonate.

Ion exchange resins are widely used in different separation, purification, and decontamination processes. The most common examples are water softening  and water purification . In many cases ion exchange resins were introduced in such processes as a more flexible alternative to the use of natural or artificial zeolites . Also, ion exchange resins are highly effective in the biodiesel filtration process.

Most typical ion exchange resins are based on crosslinked polystyrene. The actual ion exchanging sites are introduced after polymerisation. Additionally, in the case of polystyrene, crosslinking is introduced via copolymerisation of styrene and a few percent of divinylbenzene  (non-crosslinked polymers are soluble in water). Crosslinking decreases ion-exchange capacity of the resin and prolongs the time needed to accomplish the ion exchange processes but improves the robustness of the resin. Particle size also influences the resin parameters; smaller particles have larger outer surface, but cause larger head loss in the column processes.

Besides being made as bead-shaped materials, ion exchange resins are produced as membranes. The membranes, which are made of highly cross-linked ion exchange resins that allow passage of ions, but not of water, are used for eletrodialysis.

Anion resins and cation resins are the two most common resins used in the ion exchange process. While anion resins attract negatively charged ions, cation resins attract positively charged ions.