Polysaccharides are traditionally confined to the cell surface and extracellular space, whereas their nuclear roles remain unclear. Here, we show that N-glycans are present in the nucleus and modify inner nuclear membrane (INM) proteins across multiple cell types. Disruption of INM protein N-glycosylation reduces H3K9me3 in lamina-associated domains and induces genomic instability by impairing SETDB1 association with the INM. Notably, canonical ER N-glycan biosynthetic machinery contributes to INM protein N-glycosylation. Our findings reveal a previously unrecognized nuclear function of polysaccharides.