Brain invasion by glioblastoma (GBM) determines recurrence and prognosis in patients, which is, in part, attributed to increased mesenchymal transition. Here, we report evidence favoring such a role for the Pre-B-cell leukemia homebox (PBX) family member PBX3. Western blot, immunohistochemistry, qRT-PCR and datasets mining were used to determined proteins or genes expression levels. Wound-healing and transwell assays were used to examine the invasive abilities of GBM cells. Dual-luciferase reporter assays were used to determine how let-7b regulates PBX3. Chromatin-immunoprecipitation (ChIP) and rescue experiments were performed to investigate the involved molecular mechanisms. Orthotopic mouse models were used to assess the role of PBX3 in vivo. We found that PBX3 expression levels positively correlated with glioma mesenchymal markers. Ectopic expression of PBX3 promoted invasive phenotypes and triggered the expression of mesenchymal markers, whereas depletion of PBX3 reduced GBM cell invasive abilities and decreased the expression of mesenchymal markers. In addition, inhibition of PBX3 attenuated transforming growth factor-β (TGFβ)-induced GBM mesenchymal transition. Mechanistic studies revealed that PBX3 mediated GBM mesenchymal transition through activation of MEK/ERK1/2, leading to increased expression of LIN28 by c-myc. Increased LIN28 inhibited let-7b biogenesis, which then promoted the pro-invasive genes, such as HMGA2 and IL-6. Furthermore, let-7b suppressed PBX3 by directly targeting 3'-UTR of PBX3. Thus, repressed let-7b by PBX3 amplifies PBX3 signaling and forms a positive feedback loop to promote GBM mesenchymal transition. These data highlight the importance of PBX3 as a key driver of mesenchymal transition and potential therapeutic target.