The effects of ion-beam-induced damage on the normal state and superconducting properties of NbN were investigated in both single crystal and polycrystalline thin films. Three sets of films were damaged by 200-keV Ar+ ions with dosages up to 5×1016 ions/cm2. The superconducting critical temperature (Tc) of the single crystal films decreased by 20% at the highest ion dosage. Changes in the resistivity, Tc, and critical field (and its slope at Tc) suggest that Tc is suppressed by a reduction of the electron density of states at the Fermi energy due to the vacancies created by ion irradiation. The effects of ion damage are much less pronounced for the polycrystalline films. In situ grain growth was observed during the ambient temperature irradiation processes, which confirms the role of grain boundaries as a sink for ion-induced defects, hence minimizing the effects of irradiation in polycrystalline films.