Abstract <p>The magnetorheological finishing treatment (magnetorheological polishing) of the surface of water-soluble crystals of potassium dihydroorthophosphate (KDP) is studied. A nano-abrasive based on amorphous silicon dioxide obtained by modified sol-gel synthesis is introduced into magnetorheological suspension. The study of the surface roughness of optical elements after magnetorheological polishing is carried out using the profilometry. The advantages of the developed method of magnetorheological polishing of the surface of KDP single crystals include the possibility of processing large-aperture samples, as well as achieving high-quality surface treatment. Final polishing of the KDP single crystal surface with SiO₂ nano-abrasive provides a roughness value of no more than 0.6 nm.</p>