Cytosolic and nuclear proteins may be subject to both O-GlcNAcylation and proteasomal degradation. By means of activity-based profiling, we demonstrate that O-GlcNAc serine-containing peptide epoxyketones bind to the proteasome catalytic active sites and thus provide the first clear evidence that proteasomes recognize peptides post-translationally modified with a GlcNAc moiety.