A fault‐to‐fault jumping rupture was found during the 2018 6.4 Hualien earthquake that occurred in eastern Taiwan. Joint source inversion results indicated that the initial rupture started from a north–south (N‐S)–striking fault dipping to the west and propagated to the south with a high rupture speed. The rupture then jumped to the shallower east‐dipping Milun fault. Slip on the Milun fault was slow and continued for more than 10 s, which produced the largest asperity near the city of Hualien. The rupture jumped again to the Lingding fault in the northernmost Longitudinal Valley (LV). This fault‐to‐fault rupture process implied that slip on the Milun and Lingding faults was triggered by the N‐S–striking west‐dipping fault plane where the initial rupture originated. In other words, the movement on the Milun fault was passive; it was just part of a secondary branch of the major structural system in the northernmost LV. Based on this inverted source model, the static Coulomb failure stress increased along the LV, which is a crucial issue for potential seismic hazards in eastern Taiwan in the near future.