The different pocket proteins are established as negative cell cycle regulators. With regard to the repressor functions of pocket proteins in cellular senescence, studies so far have mainly focused on pRb/p105. Here, we show that in a broad range of wild-type p53-expressing human tumor cells, and in human diploid fibroblasts, Rb2/p130 is the dominating pocket protein in replicative and in accelerated senescence. Senescent cells are arrested at the transition from late G1- to early S-phase, as indicated by the absence of S- and G2-phase cyclins A and B. Expression of cyclin A and entry into S-phase resumed after RNA interference-mediated knockdown of Rb2/p130. Activation of different upstream pathways by overexpression of either p21 or p16 converged on Rb2/p130 accumulation and induced senescence. In contrast, p53- or p21-negative cells treated with DNA-damaging agents failed to accumulate Rb2/p130 and to enter senescence. Our data suggest that Rb2/p130 is a member of the p53-p21 DNA damage signaling cascade, and represents the essential pocket protein family member needed for the induction of any type of senescence.