The p57Kip2 cyclin-dependent kinase inhibitor (CDKi) has been implicated in embryogenesis, stem-cell senescence and pathologies, but little is known of its role in cell cycle control. Here, we show that p57Kip2 is targeted by the p38 stress-activated protein kinase (SAPK). Phosphorylation of p57Kip2 at T143 by p38 enhances its association with and inhibition of Cdk2, which results in cell-cycle delay upon stress. Genetic inactivation of the SAPK or the CDKi abolishes cell-cycle delay upon osmostress and results in decreased cell viability. Oxidative stress and ionomycin also induce p38-mediated phosphorylation of p57 and cells lacking p38 or p57 display reduced viability to these stresses. Therefore, cell survival to various stresses depends on p57 phosphorylation by p38 that inhibits CDK activity. Together, these findings provide a novel molecular mechanism by which cells can delay cell cycle progression to maximize cell survival upon stress.