Posttranslational modifications of clock proteins are crucial to generating proper circadian rhythms of the correct length and amplitude. Here, we show that the protein kinase CK2 (casein kinase 2) plays a role in regulating the mammalian circadian clock. We found that inhibiting CK2 activity resulted in a decrease in the amplitude and an increase in the period of oscillations in circadian gene expression. CK2 specifically bound and phosphorylated PERIOD2 (PER2) and collaborated with the protein kinase CKIepsilon to promote PER2 degradation. We also identified a CK2 phosphorylation site (serine-53) in PER2, whose phosphorylation played a role in fine-tuning circadian rhythms and regulating PER2 stability but was dispensable for the cooperative effect of CK2 and CKIepsilon. Thus, our study identifies CK2 as a regulatory element of mammalian circadian rhythms and uncovers a role for CK2 in PER2 degradation.