Reflection seismic profiles are used to constrain the late Quaternary history of folding and sedimentation in the southern Osaka Basin in southwest Japan. Seven continuous reflectors are recognized and traced throughout the offshore study area. They are correlated to lithologic boundaries in the Pleistocene Osaka Group whose ages are confirmed in four boreholes. Structure contour maps of dated horizons describe the development of N–S warping in the western portion of the study area, which had occurred at a slow rate from 1.0 to 0.4 Ma. The warping divided the sedimentary basin and controlled the distribution and facies of the Osaka Group. Microfossil assemblages suggest that growth of the warping resulted in the formation of a freshwater lake in the area of present Osaka Bay between 0.5 and 0.4 Ma. Subsequently, the southern Osaka Basin experienced accelerated subsidence produced by northwestward tilting during the Middle and Late Pleistocene.