Vertebrate centromeres are specified by the deposition of the histone H3 variant centromeric protein A (CENPA), but whether other epigenetic marks are important for centromeric chromatin function was unclear. Hori et al. now show that centromeric monomethylation of histone H4 at Lys20 (H4K20me1) is required for kinetochore assembly. H4K20me1, which is associated with transcribed regions throughout the genome, was enriched at centromeres in chicken and human cells. Moreover, immunoprecipitation data revealed that this modification occurs primarily on histone H4 of CENPA-containing nucleosomes after CENPA deposition into centromeres. Lastly, by targeting the H4K20me1 histone demethylase to centromeres, they showed that H4K20me1 is essential for the recruitment of CENPT, which is required for subsequent kinetochore assembly. Whether H4K20me1 has centromeric functions beyond CENPT recruitment remains to be determined.