The developmental regulator SOX9 is linked to cancer progression mainly as a result of its role in the regulation of cancer stem cells (CSCs). However, its activity in the differentiated cells that constitute the heterogeneous tumor bulk has not been extensively studied. In this work, we addressed this aspect in gastric cancer, glioblastoma and pancreatic adenocarcinoma. SOX9 silencing studies revealed that SOX9 is required for cancer cell survival, proliferation and evasion of senescence in vitro and tumor growth in vivo. Gain of-SOX9 function showed that high levels of SOX9 promote tumor cell proliferation in vitro and in vivo. Mechanistically, the modulation of SOX9 changed the expression of the transcriptional repressor BMI1 in the same direction in the three types of cancer, and the expression of the tumor suppressor p21CIP in the opposite direction. In agreement with this, SOX9 expression positively correlated with BMI1 levels and inversely with p21CIP in clinical samples of the different cancers. Moreover, BMI1 re-establishment in SOX9-silenced tumor cells restored cell viability and proliferation as well as decreased p21CIP in vitro and tumor growth in vivo. These results indicate that BMI1 is a critical effector of the pro-tumoral activity of SOX9 in tumor bulk cells through the repression of p21CIP. Our results highlight the relevance of the SOX9-BMI1-p21CIP axis in tumor progression, shedding novel opportunities for therapeutic development.