Understanding the variability and complex interplay of volcanism, tectonism, and sedimentation is crucial for unraveling the evolution of sedimentary basins and volcanotectonic depressions in many settings. The Miocene Janggi Basin in SE Korea, formed in association with backarc opening of the East Sea (Sea of Japan), is filled by abundant volcanic deposits intercalated with fluvio-lacustrine sedimentary deposits. Four lithostratigraphic units (Units I to IV in ascending order) are identified in the southern part of the basin, including extensive and voluminous dacitic lapilli tuffs (Units II and IV) and mixed dacitic volcaniclastic and epiclastic sedimentary deposits (Units I and III). These units suggest repetitive ignimbrite emplacement onto a braided-stream to lacustrine environment. Paleoflow measurements show that the epiclastic conglomerates were deposited by a north-flowing fluvial system that debouched from a drainage area to the south of the basin, whereas the ignimbrites were introduced from the east. Because of the separate source areas, the epiclastic and volcaniclastic depositional processes did not interfere with each other, resulting in discrete alternations of epiclastic and mostly primary volcaniclastic deposits. The studied succession also lacks an aggradation–degradation cycle, which results from the alternation of periods of rapid volcaniclastic sedimentation and relatively longer periods of fluvial incision. The volcaniclastic and epiclastic deposits are therefore interpreted to have accumulated in rapid succession, in association with rapid basin subsidence and contemporaneous ignimbrite-forming eruptions. The studied succession can thus be regarded as a syntectonic sequence, in which the stratal geometry was significantly modulated by tectonostratigraphic controls, with overprints of syntectonic contemporaneous volcanism.