Disruptor of telomeric silencing (Dot1p) is an exquisitely conserved histone methyltransferase, and is the sole enzyme responsible for H3K79 methylation in the budding yeast, Saccharomyces cerevisiae. It has been shown to be highly phosphorylated in vivo; however, the upstream kinases that act on Dot1p are unknown in yeast and all other eukaryotes. Here, we used in vitro and in vivo kinase discovery approaches to show that the mitogen-activated protein kinase Hog1p is a bona fide kinase of Dot1p methyltransferase. In in vitro kinase assays, Hog1p phosphorylates Dot1p at multiple sites, including at several proline-adjacent sites that are consistent with its known substrate preference. Hog1p kinase activity was specifically enhanced at these proline-adjacent sites on Dot1p upon its activation by the osmostress-responsive kinase, Pbs2p. We then show that genomic deletion of HOG1 markedly reduces phosphorylation at specific sites on Dot1p in vivo, thus providing evidence for Hog1p kinase activity on Dot1p in budding yeast cells. Phenotypic analysis of knockout and phosphosite mutant yeast strains revealed the importance of Hog1p-catalysed phosphorylation of Dot1p for cellular responses to ultraviolet-induced DNA damage. This kinase-substrate relationship is conserved in mammalian systems, where human DOT1L (ortholog of Dot1p) can be phosphorylated by the proline-directed kinase p38β/MAPK11 (ortholog of Hog1p) at multiple sites in vitro. Taken together, our findings establish Hog1p and p38β/MAPK11 as the first kinases known to act on H3K79 methyltransferase enzymes in any eukaryotic species.