A new single phase of composition Bi3NbO7 has been isolated in the binary system Bi2O3-Nb2O5. X-ray single crystal diffraction studies have shown that Bi3NbO7 crystallizes in the cubic system, space group Fm (3) over bar, with unit-cell parameter a = 5.4788(9) Angstrom. The structure has been determined, and the final reliability index is R = 0.027. Bi3NbO7 exhibits a defect fluorite-type structure with 12.5% of anion vacancies and appears to show a disorder in both anionic and cationic lattices. The study by electron diffraction and high resolution electron microscopy has shown the presence of an incommensurate cubic lattice. Bi3NbO7 shows good behavior as an ionic conductor up to 875 degrees C without any phase transition as deduced from impedance measurements.