Histone H3 lysine 56 acetylation (H3K56Ac) has recently been identified and shown to be important for genomic stability in yeast. However, whether or not H3K56 acetylation occurs in mammals is not clear.  Here, we report that H3K56Ac exists in mammals.  Mammalian H3K56Ac requires the histone chaperone Asf1 and occurs mainly at the S phase in unstressed cells. Moreover, SIRT1, which is a mammalian member of sirtuin family of NAD+-dependent deacetylases, regulates the deacetylation of H3K56.  We further showed that proper H3K56 acetylation is critical for genomic stability and DNA damage response.  These results establish the existence and functional significance of H3K56Ac in mammals and identify two regulators of this modification.