Undoped and doped Nb$_2$O$_5$ thin films were successfully deposited on silicon wafer by sputtering technique from high purity Nb$_2$O$_5$ and Tb$_4$O$_7$ targets and annealed by rapid thermal annealing between 400 °C and 900 °C. Chemical and structural investigations of undoped and Tb-doped Nb$_2$O$_5$ films were made, leading to the determination of stoichiometry and identification of phase transformations with increasing annealing temperature and Tb$^{3+}$ ion content. The optical properties' analysis (spectrophotometry UV–Vis-NIR, spectroscopic ellipsometry) of those undoped and doped Nb$_2$O$_5$ films led us to the determination of refractive index and electronic properties evolution with annealing temperature and Tb$^{3+}$ ion content. The structural evolution was confirmed by analysis of Raman spectra of undoped and doped Nb$_2$O$_5$ films. The photoluminescence properties of Tb$^{3+}$ ion-doped Nb$_2$O$_5$ thin films were also studied. The correlation between the photoluminescence of Tb$^{3+}$ ion and the electronic properties of the Nb$_2$O$_5$ host matrix is highlighted. The relationship between the structural modifications, the electronic properties of the host matrix, and the photoluminescence of Tb$^{3+}$ ion is established.