Taiwan, a seismically active zone, has historically experienced many devastating earthquakes such as the 1999 Mw 7.6 Chi‐Chi event. Understanding seismic risk in this highly populated region requires harnessing all paleoseismological indicators in the recent tectonic evolution of the island. Fault pseudotachylytes, widely regarded as earthquake fossils, provide a wealth of information on dynamic processes during seismic rupture. A new pseudotachylyte locality, discovered in Eastern Taiwan along the Hoping River in the Tananao metamorphic complex, provides an exceptional opportunity to constrain a Pliocene‐Pleistocene (<4.1–3.0 Ma) seismic rupture. With a net displacement of 220 mm, seismic slip produced veins of frictional melt correspond-ing to a Mw 6.4 ± 0.4 earthquake. This measured displacement is compared to that inferred from vein thickness. Detailed microstructural observations reveal that only portions of the veins display criteria consistent with a melt origin, the rest being ultracataclasites. Outcrop‐scale observations show that seismic slip took place along a dip‐slip direction with a normal kinematic, consistent with the exhumation of the metamorphic complex. Our approach resolved all seismic rupture parameters for an ancient earthquake from a single pseudotachylyte vein. Finally, this study suggests that eastern Taiwan Pliocene‐Pleistocene tectonics might have been an exhumation‐related extension.