Ferroelectric Na0.5Bi4.5Ti4O15 (NaBTi) and donor Nb-doped Na0.5Bi4.5Ti3.94Nb0.06O15 (NaBTiNb) thin films were prepared on Pt(111)/Ti/SiO2/Si(100) substrates using a chemical solution deposition method. The doping with Nb5+-ions leads to tremendous improvements in the ferroelectric properties of the NaBTiNb thin film. Room-temperature ferroelectricity with a large remnant polarization (2Pr) of 64.1 μC/cm2 and a low coercive field (2Ec) of 165 kV/cm at an applied electric field of 475 kV/cm was observed for the NaBTiNb thin film. The polarization fatigue study revealed that the NaBTiNb thin film exhibited good fatigue endurance compared with the NaBTi thin film. Furthermore, the NaBTiNb thin film showed a low leakage current density, which was 1.48 × 10-6 A/cm2 at an applied electric field of 100 kV/cm.