PATZ1 is a transcriptional factor functioning either as an activator or a repressor of gene transcription depending upon the cellular context. It appears to have a dual oncogenic/anti-oncogenic activity. Indeed, it is overexpressed in colon carcinomas, and its silencing inhibits colon cancer cell proliferation or increases sensitivity to apoptotic stimuli of glioma cells, suggesting an oncogenic role. Conversely, the development of B-cell lymphomas, sarcomas, hepatocellular carcinomas and lung adenomas in Patz1-knockout (ko) mice supports its tumour suppressor function. PATZ1 role in mouse lymphomagenesis is mainly because of the involvement of PATZ1 in BCL6-negative autoregulation. However, this does not exclude that PATZ1 may be involved in tumorigenesis by other mechanisms. Here, we report that PATZ1 interacts with the tumour suppressor p53 and binds p53-dependent gene promoters, including those of BAX, CDKN1A and MDM2. Knockdown of PATZ1 in HEK293 cells reduces promoter activity of these genes and inhibits their expression, suggesting a role of PATZ in enhancing p53 transcriptional activity. Consistently, Patz1-ko mouse embryonic fibroblasts (MEFs) show decreased expression of Bax, Cdkn1a and Mdm2 compared with wild-type (wt) MEFs. Moreover, Patz1-ko MEFs show a decreased percentage of apoptotic cells, either spontaneous or induced by treatment with 5-fluorouracil (5FU), compared with wt controls, suggesting a pro-apoptotic role for PATZ1 in these cells. However, PATZ1 binds p53-target genes also independently from p53, exerting, in the absence of p53, an opposite function on their expression. Indeed, knockdown of PATZ1 in p53-null osteosarcoma cells upregulates BAX expression and decreases survival of 5FU-treated cells, then suggesting an anti-apoptotic role of PATZ1 in p53-null cancer cells. Therefore, these data support a PATZ1 tumour-suppressive function based on its ability to enhance p53-dependent transcription and apoptosis. Conversely, its opposite and anti-apoptotic role in p53-null cancer cells provides the perspective of PATZ1 silencing as a possible adjuvant in the treatment of p53-null cancer.