Oxide nanoparticles have numerous potential applications in medicine such as carriers for therapeutic drugs, contrast agents for bio-imaging and targeting agents for tumors. Oxide nanoparticles may also have an inherent cytotoxicity towards cancer cells, as recently found for cerium oxide. KNbO3 nanoparticles have a combination of low toxicity and nonlinear optical properties which make them attractive for use as a bio-imaging material. However, little is known yet about the cytotoxicity of KNbO3 particles towards cancerous cells. In the present work, the cytotoxicity of KNbO3 particles to normal and prostate cancer cell lines is studied. The mixed oxide method is used to prepare KNbO3 powder. Using dynamic light scattering the mean particle diameter of the KNbO3 powder is found to be ∼500 nm. X-ray diffraction, Fourier transform infra-red spectroscopy and Raman scattering spectroscopy are used to examine the structure of the KNbO3 powder. Powder morphology is examined using scanning electron microscopy. MTT assays of EA.hy926, PC-3 and DU-145 cell lines are carried out to study cell-specific cytotoxicity. KNbO3 sub-micron particles are found to have low toxicity to PC-3 cells, moderate toxicity to EA.hy926 cells and high toxicity to DU-145 cells. A new avenue towards the treatment of prostate cancer may be opened by the cell-specific cytotoxicity of KNbO3.