Reorganization of the cortical actin cytoskeleton at mitotic entry is essential to increase membrane tension for cell rounding. This spherical shape is necessary for the biogenesis and organization of the mitotic spindle. Proteins of the Ezrin, Radixin, Moesin (ERM) family play essential roles in mitotic morphogenesis by linking actomyosin forces to the plasma membrane. While ERMs drive metaphase cell rounding, the cell-cycle signals that prompt their conformational activation in mitosis are unknown. We screened a library of small molecules using novel ERM biosensors and we unexpectedly found that drugs that disassemble microtubules promote ERM activation. Remarkably, cells disassemble their interphase microtubules while entering mitosis. We further discovered that this disassembly of microtubules acts as a cell-cycle signal that directs ERM activation and metaphase cell rounding. We show that GEF-H1, a Rho-GEF inhibited by microtubule binding, acts downstream of microtubule disassembly to activate ERMs via RhoA and its kinase effector SLK. In addition, we demonstrate that GEF-H1 and Ect2, another Rho-GEF responsible for the generation of mitotic actomyosin forces, act together to drive metaphase ERM activation and cell rounding. In summary, we report microtubule disassembly as a cell cycle signal that triggers a signaling network ensuring that actomyosin forces are efficiently integrated at the plasma membrane to promote cell rounding at mitotic entry.