Radiative lifetimes of 17 excited levels of Nbi, in the energy range 27400-47700 cm-1 (5p y 6D°9/2, 5p x 6D°7/2, 5p w 4G°7/2,9/2,11/2, 5s5p v 4D°1/2,3/2,5/2,7/2, 5s6p n 4D°1/2,3/2,5/2,7/2, 5s6p o 4F°3/2,5/2,7/2,9/2), have been measured. For 15 of these levels, the lifetimes are obtained for the first time. The lifetimes were measured using the time-resolved laser-induced fluorescence technique and the experiments are complemented by a theoretical investigation using a relativistic Hartree-Fock method including core polarization. By combining the experimental lifetimes and the calculated branching fractions, we obtain transition probabilities for the individual de-excitation channels from the investigated levels