Epitaxial potassium niobate (KNbO3) thin films have been deposited on KTaO3 (100), MgAl2O4 (100), and MgO (100) substrates using ion‐beam sputter deposition. X‐ray‐diffraction results show that KNbO3 films have orthorhombic (110) orientation on all three substrates. Rutherford backscattering channeling analysis of KNbO3 films on KTaO3, MgAl2O4, and MgO exhibits minimum scattering yields (χmin) of 7%, 9%, and 18% for the Nb peak, respectively. This illustrates how the quality of epitaxy improves as the lattice mismatch decreases. Prism‐coupling measurements reveal near‐bulk refractive indices of about 2.27 for TE modes and 2.22 for TM modes for films on each substrate.