Glioblastoma (GB) is the most malignant and aggressive primary brain tumor in adults, characterized by rapid growth, diffuse infiltration, and strong resistance to treatments. As a result, most patients inevitably experience tumor recurrence, and median survival remains around 15 months. In this context, Tumor Treating Fields (TTFields) have emerged as an innovative and noninvasive therapy. Although clinical use of TTFields has shown survival benefits, most patients relapse, highlighting adaptive mechanisms that enable GB cells to survive prolonged treatment. To better understand the molecular basis of TTFields resistance, we created an experimental model that mimics clinical exposure by applying long-term TTFields stimulation to patient-derived GB cultures representing different molecular subtypes. Initial responses to therapy varied among cell lines; however, after extended exposure, all resistant groups developed a typical mesenchymal-like phenotype, indicating a shared adaptive response. Transcriptomic analysis of paired patient samples before and after treatment further showed that TTFields-treated tumors tend to upregulate pathways related to myelin formation, consistent with a mesenchymal-like phenotype. While these pathways reflect a shift toward a more invasive and therapy-resistant state, they offer limited options for direct drug targeting. We identified the transmembrane form of Chloride Intracellular Channel 1 (tmCLIC1) as a potential target. tmCLIC1 has been linked to GB stem cell metabolism, regulation of oxidative stress, and tumor growth, suggesting it plays a critical role in maintaining resistance to TTFields. Based on these findings, we tested the therapeutic benefit of concomitant treatment of TTFields and metformin, a well-known antidiabetic drug that inhibits tmCLIC1 in GB. Remarkably, this combination produced a strong synergistic effect, significantly reducing GB cell viability in vitro and slowing tumor growth in vivo. These results show that prolonged TTFields exposure pushes GB cells toward a mesenchymal-like, therapy-resistant phenotype and highlight tmCLIC1 as a key player in this adaptive process. Targeting tmCLIC1 with metformin offers a promising strategy to sensitize resistant cells and improve the effectiveness of TTFields. This work provides a mechanistic framework for developing combination therapies to overcome treatment resistance and enhance long-term outcomes for GB patients.