Sirt6 is a histone deacetylase with NAD+-dependent activity. Sirt6 has been shown as a tumor suppressor partially via inhibiting the expression of c-Myc target genes and ribosome biogenesis. However, how to regulate Sirt6 activity is largely unknown. In this study, we identify that Sirt6 can be modified by small ubiquitin-like modifier. Sirt6 SUMOylation deficiency specifically decreases its deacetylation of H3K56 but not H3K9 in vivo. Mechanistically, we find that SUMOylation deficiency decreases Sirt6 binding with c-Myc, decreasing Sirt6 occupancy on the locus of c-Myc target genes. Therefore, Sirt6 SUMOylation deficiency reduces its deacetylation of H3k56 and its repression of c-Myc target genes. Moreover, Sirt6 SUMOylation deficiency reduces its suppression of cell proliferation and tumorigenesis. Thus, these results reveal that SUMOylation has an important role in regulation of Sirt6 deacetylation on H3K56, as well as its tumor suppressive activity.