The cyclin‐dependent kinase (CDK) inhibitor p21 Waf1/Cip1/Sdi1 was identified initially as a gene induced in senescent cells and itself has been shown to cause permanent growth arrest/senescence. Reactive oxygen species (ROS), a byproduct of oxidative processes, can also induce an irreversible growth arrest similar to senescence. Here we show that p21 increased intracellular levels of ROS both in normal fibroblasts and in p53‐negative cancer cells. N‐acetyl‐l‐cysteine, an ROS inhibitor, rescued p21‐induced senescence, showing that ROS elevation is necessary for induction of the permanent growth arrest phenotype. p16 Ink4a, a CDK4‐and CDK6‐specific inhibitor, failed to increase ROS levels, and cell cycle arrest induced by p16 was reversible following its down‐regulation, demonstrating the specificity of this p21 effect. A p21 mutant that lacked the ability to bind proliferating cell nuclear antigen (PCNA) retained the ability to induce both ROS and permanent growth arrest. All of these findings establish that p21 mediates senescence by a mechanism involving ROS accumulation which does not require either its PCNA binding or the CDK inhibitory functions shared with p16.