Injury, tumors, ischemia, and lesions in the cerebellum show the involvement of this region in human speech. The association of the cerebellum with learned birdsong has only been identified recently. Cerebellar dysfunction in young songbirds causes learning disabilities, but its role in adult songbirds has not been established. The aim of this study was to investigate the role of the deep cerebellar nuclei (DCN) in adult birdsong. We created bilateral excitotoxic lesions in the DCN of adult male zebra finches (Taeniopygia guttata) and recorded their songs for up to 4 months. Using magnetic resonance imaging (MRI) and immunohistochemistry, we validated the lesion efficacy. We found that the song duration significantly increased from 14 weeks post-op; the increase in duration was caused by a greater number of introductory notes as well as a greater number of syllables sung after the introductory notes. On the other hand, the motif duration decreased from 8 weeks after DCN lesions were induced, which was due to faster singing of syllables, not changes in inter-syllable interval length. DCN lesions also caused a decrease in the fundamental frequency of syllables. In summary, we showed that DCN lesions influence the temporal and acoustic features of birdsong. These results suggest that the cerebellum influences singing in adult songbirds.