A mixed bismuth layer-structured compound, Bi7Ti4NbO21, has been prepared by the conventional solid state reaction. It showed an orthorhombic symmetry with a = 5.4428, b = 5.4043 and c = 29.041 Å by X-ray powder diffraction analysis. The hysteresis loops as a function of temperature were observed with a standardized ferroelectric test system. The remanent polarization and the coercive field of the material at 140°C were 14.06 µC/cm2 and 78.6 kV/cm, respectively. Thermal dependence of dielectric permittivity showed two-phase transitions at around 670°C and 845°C, which were also investigated by TSC and DSC. Finally, piezoelectric properties were obtained with a piezoelectric coefficient d33 = 10 pC/N. It was observed that Bi7Ti4NbO21 underwent a ferroelectric–paraelectric phase transition at 845°C by depolarization experiments.