Cellular senescence is an irreversible growth arrest and is presumed to be a natural barrier to tumor development. Like telomere shortening, certain defects in chromosome integrity can trigger senescence; however, the roles of centromere proteins in regulating commitment to the senescent state remains to be established. We examined chromatin structure in senescent human primary fibroblasts and found that CENP-A protein levels are diminished in senescent cells. Senescence-associated reduction of CENP-A is caused by transcriptional and posttranslational control. Surprisingly, forced reduction of CENP-A by short-hairpin RNA was found to cause premature senescence in human primary fibroblasts. This premature senescence is dependent on a tumor suppressor, p53, but not on p16(INK4a)-Rb; the depletion of CENP-A in p53-deficient cells results in aberrant mitosis with chromosome missegregation. We propose that p53-dependent senescence that arises from CENP-A reduction acts as a "self-defense mechanism" to prevent centromere-defective cells from undergoing mitotic proliferation that potentially leads to massive generation of aneuploid cells.