O-GlcNAcylation is an abundant posttranslational modification in the brain implicated in human neurodegenerative diseases. We have exploited viable null alleles of the enzymes of O-GlcNAc cycling to examine the role of O-GlcNAcylation in well-characterized Caenorhabditis elegans models of neurodegenerative proteotoxicity. O-GlcNAc cycling dramatically modulated the severity of the phenotype in transgenic models of tauopathy, amyloid β-peptide, and polyglutamine expansion. Intriguingly, loss of function of O-GlcNAc transferase alleviated, whereas loss of O-GlcNAcase enhanced, the phenotype of multiple neurodegenerative disease models. The O-GlcNAc cycling mutants act in part by altering DAF-16-dependent transcription and modulating the protein degradation machinery. These findings suggest that O-GlcNAc levels may directly influence neurodegenerative disease progression, thus making the enzymes of O-GlcNAc cycling attractive targets for neurodegenerative disease therapies.