Joint seismic and geodetic analyses revealed that the 2025 Mw7.7 Myanmar earthquake ruptured ~530 km along the Sagaing Fault, with a sustained supershear rupture extending ~450 km on its southern branch. Far-field Mach waves and near-field ground motion confirmed the supershear nature. This exceptionally long supershear rupture caused severe building collapse and soil liquefaction, as observed in satellite imagery, offering insights into the damage potential of such ruptures in urban areas. The sustained supershear propagation was facilitated by fault’s linear geometry, prolonged interseismic quiescence, favorable energy ratio, and pronounced bimaterial contrasts across the fault interface. These findings underscore the roles of fault structure, stress accumulation, and material contrasts in governing rupture dynamics, demonstrating that large-scale supershear propagation can occur in complex continental fault systems.