The electronic and structural properties of a series of triniobium oxide clusters, Nb3On- and Nb3On (n = 3-8), are investigated using photoelectron spectroscopy (PES) and density functional theory (DFT) calculations. PES spectra are obtained for Nb3On- (n = 3-8) at various photon energies and are used to compare with the DFT calculations. A trend of sequential oxidation is observed as a function of O content until Nb3O8-, reaching the highest oxidation state of Nb. DFT calculations are performed to search for the lowest energy structures for both the anionic and neutral clusters. For Nb3O3-, the three O atoms are shown to prefer the bridging sites of a triangular Nb-3, leading to two nearly degenerate cyclic structures of C-s symmetry. The next three O atoms from n = 4-6 each occupy a terminal site directly bonded to Nb, resulting in a symmetric Nb3O6- with C-3v, symmetry and a low-lying isomer of C-s symmetry. The seventh O atom is bonded to two Nb atoms forming a double bridge, whereas the eighth O atom is bonded to a terminal site so that in Nb3O8- each Nb atom reaches its maximum oxidation state of +5. The structures and electronic states for the triniobium oxide clusters are significantly different from the corresponding tritantalum oxide clusters, in particular, for Nb3O3-, Nb3O5-, and Nb3O7-. Molecular orbital analyses are performed to elucidate the chemical bonding and the electronic and structural evolution in these triniobium oxide clusters.