Abstract: Impregnation of pores of porous silicon (PS) by Lanthanum Fluoride (LaF₃) using a novel one-step chemical bath technique, and application of the LaF₃-impregnated porous silicon (PS) structure (LaF₃/PS) as a potentiometric fluoride ion sensor have been investigated in this article. The impregnation of LaF₃ inside the pores of porous silicon was achieved using a chemical bath technique developed by this group. The Scanning Electron Microscopy (SEM) and EDX on the cross-section of LaF₃/PS/Si structure confirmed the LaF₃ film deposition inside the pores of PS. The heterostructure of LaF₃/PS/Si was investigated as fluoride ion (F^-) sensor in aqueous medium. The high specific area of PS was taken as the key subject to investigate the high fluoride sensitivity of the LaF₃/PS structure in aqueous medium. When experimented with home made fluoride solution having various concentrations the annealed LaF₃/PS/Si structure was found to detect the fluoride ion in aqueous solution. Its response was found linear in the fluoride concentration range of 2.28~4.28 pF. As capacitive sensor the overall fluoride sensitivity was found to be over-Nernstian (400 mV/pF). The experimental results indicate that LaF₃-impregnated porous silicon structure (LaF₃/PS) can be used as a high-sensitive fluoride-ion sensor in aqueous medium.