Two earthquakes ( 5.1 and 5.5) ruptured branches of the Yangsan fault system in Gyeongju, South Korea, on 12 September 2016. Aftershocks, including a notable 4.3 earthquake on 19 September 2016, were clustered around the epicenters of the first two events. The 5.5 earthquake is considered the largest earthquake in South Korea to have occurred during the modern instrumental recording period since 1978. Although there is no apparent surface rupture, these earthquakes have greatly shaken South Korea, leaving both physical and societal impacts. In this study, we determine the source mechanism and rupture directivity using regional seismic‐waveform data to understand the earthquake source processes. Based on the waveform inversion, we report that the mainshock ( 5.5 event) is a strike‐slip event with two nodal planes 117°/84°/21° and 24°/69°/173° at a depth of 14 km. The inversion also demonstrates that the mainshock event ruptured against the 24° seismogenic fault plane to the south‐southwest, with a rupture length of . This rupture propagation direction agrees well with the spatial distribution of relocated aftershock events and reported seismic intensities.