Using borehole strainmeters, we detected a 13-day long slow slip event on the Longitudinal Valley Fault, Taiwan. It is located between 8 to 15 km depth and has an equivalent moment magnitude of 5.5. The slow event has likely been promoted by the significant Coulomb stress changes (∼ + 1 MPa) imparted by a combination of coseismic and postseismic slip of the M w 6.8 Chengkung earthquake. Besides, insignificant coseismic slip is observed in the slow event region, suggesting that the latter could have acted as a barrier during the Chengkung earthquake. We also found a spatiotemporal correlation between the slow event and a cluster of repeating microearthquakes, suggesting aseismic slip as a possible driven mechanism of repeating ruptures. These results highlight the complex interplay between seismic and aseismic processes along the fault.