The Deokpori Thrust, juxtaposing the early Paleozoic Yeongwol‐type Joseon Supergroup (Yeongwol Group) against the Duwibong‐type Joseon Supergroup (Taebaek Group) in the Okcheon belt, is an important regional structure in the Phanerozoic tectonic evolution of Korea. Although the initiation of the thrust has recently been conjectured to occur during the Middle Jurassic Daebo tectonic event in a continental magmatic arc setting, its movement history has never been studied. We report timing of brittle reactivation of the thrust obtained from K‐Ar dating of fault gouges in the core of the Deokpori Thrust zone. K‐Ar illite age data of fault gouges suggest reactivation of the thrust at ca. 72 Ma after a contact metamorphism around 94 Ma. Our result is the first documentation of the reactivation of a major Jurassic ‘Daebo’ structure and the reactivation may represent a significant contractional tectonic event during the Late Cretaceous in the Korean Peninsula.