Hypoxia is a common feature of solid tumors and represents a critical factor in their progression and responsiveness to chemotherapy and radiotherapy. We now report that hypoxic exposure of colon cancer cells decreased the protein levels of the cell cycle-controlling phosphatase Cdc25A. Hypoxia decreased the mitotic population and caused S-phase arrest in these cells. Suppression of Cdc25A was phosphatase family member-specific, as a similar decrease was not observed with closely related Cdc25B or Cdc25C phosphatases. Pharmacological and genetic blockade of Chk1 and Chk2 failed to inhibit the hypoxia-mediated loss of Cdc25A, indicating this process was not regulated by a traditional ATM/ATR checkpoint response. In addition, hypoxia did not affect ectopically expressed Cdc25A levels suggesting independence from an increase in proteasomal degradation. Cdc25A mRNA levels also decreased in human colon cancer cells 24 hr after hypoxia supporting a mechanistic role for decreased Cdc25A expression or mRNA stability. The reduction in Cdc25A mRNA and protein was dependent on the cyclin-dependent kinase inhibitor p21 and miR-21, which were upregulated in HCT116 colon cancer cells during hypoxia. These results reveal previously unknown mechanisms for the transient suppression of Cdc25A, providing a coordinated and fundamental adaptive change that may be exploited by cancer cells conferring proliferative and survival advantages.