SIRT6 is a member of the Sir2 (Silent Information Regulator-2) family of genes, which regulate fundamental processes in aging and lifespan control in multiple organisms.1-3 SIRT6 deficiency in mice results in genomic instability, metabolic defects, and degenerative phenotypes associated with aging.1 Previously, we showed that SIRT6 deacetylates lysine 9 on the N-terminal tail of histone H3 (H3K9Ac) to modulate telomeric chromatin and gene expression.2,3 Here, we identify a second substrate of SIRT6 at chromatin, lysine 56 on the globular core of histone H3 (H3K56Ac). We show that SIRT6 deacetylates H3K56Ac in vitro and in cells, and identify a physiologic role for this activity in maintaining dynamic changes of H3K56 acetylation levels at telomeric chromatin over the cell cycle. Together, these findings provide the first analysis of how H3K56Ac levels are dynamically regulated at human telomeres in response to a mammalian SIRT.