A new modified ion-selective electrode with membranes of LaF₃ single crystals with different internal contacts (solid steel or electrolyte) and with Fe_xO_y nanoparticles as loading was developed. The best response characteristic with linear potential change was found in the fluoride concentration range from 10⁻¹ to 3.98 × 10⁻⁷ M. The detection limit for the electrolyte contact was determined at 7.41 × 10⁻⁸ M with a regression coefficient of 0.9932, while the regression coefficient for the solid contact was 0.9969. The potential change per concentration decade ranged from 50.3 to 62.4 mV, depending on whether the contact was solid or electrolytic. The prepared modified electrode has a long lifetime, as well as the possibility of application in different positions (solid contact), and it can also be used for the determination of iron ions. The electrode characterization was performed with scanning electron microscopy and elemental analysis with the technique of electron-dispersive X-ray spectroscopy.