Background: Medulloblastoma (MB), the most common malignant pediatric brain tumor, is often incurable upon recurrence, largely driven by treatment-resistant quiescent cells. While quiescent SHH MB populations have been identified, the mechanisms driving their chemoresistance remain unclear. Here, we investigate the role of the cell cycle inhibitor p57 in inducing quiescence and show that dexamethasone, widely used in MB management, promotes p57-mediated quiescence, potentially reducing treatment efficacy. Methods: To assess p57’s role, we introduced a TMP-inducible p57 construct into Ptch1+/- SHH MB cells and treated them with vincristine. We also treated Ptch1+/- SHH MB cells with dexamethasone and quantified p57 levels and cell cycle states using high-throughput immunofluorescence imaging. Results: In culture, nuclear p57 was enriched in Sox2+ and Nestin+ stem-like SHH MB cells relative to rapidly-cycling Atoh1+ cells. Stabilizing p57 with TMP increased G0-phase cells six-fold, exhibiting survival to vincristine doses that caused complete cell death in controls. Dexamethasone treatment increased nuclear p57 by 40% and G0-phase cells by 15% in Ptch1+/- cells, while doubling G0-phase cells in Ptch1+/-;Trp53-/- cells. Conclusions: These findings suggest dexamethasone promotes p57-mediated quiescence, potentially contributing to chemoresistance in SHH MB. This raises critical concerns about the use of dexamethasone in MB treatment, as it may inadvertently enhance tumor recurrence.