In recent years, the intracellular reactive oxygen species (ROS) levels have gained increasing attention as a critical regulator of cellular proliferation. We investigated the hypothesis that manganese superoxide dismutase (MnSOD) activity regulates proliferative and quiescent growth by modulating cellular ROS levels. Decreasing MnSOD activity favored proliferation in mouse embryonic fibroblasts (MEF), while increasing MnSOD activity facilitated proliferating cells’ transitions into quiescence. MnSOD (+/–) and (–/–) MEFs demonstrated increased superoxide steady‐state levels; these fibroblasts failed to exit from the proliferative cycle, and showed increasing cyclin D1 and cyclin B1 protein levels. MnSOD (+/–) MEFs exhibited an increase in the percentage of G₂ cells compared to MnSOD (+/+) MEFs. Overexpression of MnSOD in MnSOD (+/–) MEFs suppressed superoxide levels and G₂ accumulation, decreased cyclin B1 protein levels, and facilitated cells’ transit into quiescence. While ROS are known to regulate differentiation and cell death pathways, both of which are irreversible processes, our results show MnSOD activity and, therefore, mitochondria‐derived ROS levels regulate cellular proliferation and quiescence, which are reversible processes essential to prevent aberrant proliferation and subsequent exhaustion of normal cell proliferative capacity. These results support the hypothesis that MnSOD activity regulates a mitochondrial ‘ROS‐switch’ favoring a superoxide‐signaling regulating proliferation and a hydrogen peroxide‐signaling supporting quiescence.