The cyclin-dependent kinase 1 (Cdk1) represents an ancient cell cycle kinase that has been conserved from yeast to humans. Cdk1 is the only essential mammalian Cdk, which drives cytokinesis by phosphorylating a large number of cellular proteins. To uncover additional functions of Cdk1, we generated a knock-in strain of mice expressing an analog-sensitive version of Cdk1 in place of wild-type Cdk1. These mice and cells derived from them allow us to investigate Cdk1 function in essentially any compartment and at any stage of development. In our study, we focused on embryonic stem (ES) cells, as this cell type expresses particularly high levels of Cdk1. Very unexpectedly, we found that in ES cells the majority of Cdk1 substrates are localized on chromatin. Cdk1 phosphorylates a large number of proteins involved in epigenetic regulation, including writers and erasers of all major histone marks. Consistent with this finding, inhibition of Cdk1 altered histone modification status of ES cells. High levels of Cdk1 in ES cells (and in induced pluripotent stem cells) phosphorylate and partially inactivate Dot1l, the histone H3 lysine 79 methyltransferase responsible for placing activating H3K79 marks on gene bodies. Decrease of Cdk1 activity during ES cell differentiation de-represses Dot1l, thereby allowing coordinated expression of differentiation genes. These analyses indicate that Cdk1 functions to maintain the epigenetic identity of ES cells.