The frictional properties of clay‐rich fault material collected from a megasplay fault within the Nankai accretionary complex were examined in frictional experiments performed at a normal stress of 5 MPa under water‐saturated conditions for slip velocities from 0.0026 to 260 mm/s with >250 mm of displacement. Our results reveal that the fault material can show two stages of velocity weakening behavior: weakening at slow velocities (v < 2.6 mm/s), characterized by a small degree of friction velocity dependence (the absolute value of (a − b) is typically <0.005), and a dramatic weakening at high velocities (v > 26 mm/s). Such a process of fault weakening may provide important constraints on models of faulting along a megasplay fault. At slip velocities from 0.026 to 2.6 mm/s, there exist both velocity‐weakening and velocity strengthening fault materials. The frictional coefficient values, m, for slow slip velocities (v = 0.26 mm/s) are relatively low (m = 0.28-0.35) for velocity‐strengthening samples compared with velocity‐weakening samples (m = 0.38-0.49). Microstructural analyses reveal that velocity‐strengthening samples show homogeneous deformation textures in which the entire gouge layer is deformed, whereas velocity‐weakening materials show evidence of shear localization in which deformation is concentrated along narrow subsidiary shears