Much in vivo evidence indicates that cyclooxygenase-2 (COX-2) is deeply involved in tumorigenesis. Although it has been proposed that COX-2-derived pro-inflammatory prostanoids mediate the tumorigenic activity of COX-2, the tumorigenic mechanisms of COX-2 are not yet fully understood. Here, we investigated the mechanism by which COX-2 causes transformation from normal cells to malignant cells by using normal murine or human cells. We found that COX-2 inhibits the pro-senescent function of p53 under oncogenic RAS activation, by which it prevents oncogene-induced senescence (OIS) and induces neoplastic transformation. We also found that COX-2 physically interacts with p53 in the nucleus under oncogenic RAS activation, and that this COX-2-p53 interaction rather than the catalytic activity is involved in the COX-2-mediated inhibition of the pro-senescent function of p53 and OIS, and induction of neoplastic transformation. These findings strongly suggest that the oncogenic property of COX-2 is closely related to its ability to inactivate p53 under strong mitogenic signals, and that aberrant activation of the COX-2/a mitogenic oncogene combination can be a potent driving force for tumorigenesis. This study might contribute to our understanding of the molecular basis for the tumorigenic activity of COX-2 and the development of novel anti-tumor drugs targeting COX-2-p53 interactions.