Extension and rifting of the lithosphere is fundamental to the evolution of the continents, but the mechanism by which the lithosphere thins under extension remains enigmatic. Using a new dense array of magnetotelluric data from the rifted margin of southeast China, we resolve the three‐dimensional electrical resistivity structure of the lithosphere to constrain the process of rifting and thinning. Our results reveal a brittle‐ductile transition zone in the Cathaysia Block, featuring low electrical resistivity and low seismic velocity at 15‐ to 20‐km depth. A southeast directed dip is resolved for the Jiangshan‐Shaoxing Fault—a suture zone originally formed by the Neoproterozoic collision of Yangtze and Cathaysia Blocks. This boundary fault had been reactivated to different extents by the Early Paleozoic and Early Mesozoic intracontinental orogenies of south China. Its present structural style, however, was interpreted as a lithospheric‐scale detachment fault that acted during the Late Mesozoic extension and rifting. Given the asymmetries of topography, electrical resistivity, Bouguer gravity anomaly, and Late Mesozoic volcanism across the Gan‐Hang Rift, we propose an asymmetric simple shear extension model for the south China rift system. Water content of up to 0.1 wt% and melt fraction of up to 1% are estimated at 70‐km depth beneath the central Wuyi Mountains, suggesting hydration and partial melting of the mantle lithosphere. The hydration weakening of the mantle lithosphere promoted both the gravitational instability and convective removal of the lowermost lithosphere in south China.