Stratigraphic, provenance, and subsidence analyses suggest that by the Middle to Late Jurassic a foreland basin system was active in northwestern Montana (United States). U-Pb ages of detrital zircons and detrital modes of sandstones indicate provenance from accreted terranes and deformed miogeoclinal rocks to the west. Subsidence commenced ca. 170 Ma and followed a sigmoidal pattern characteristic of foreland basin systems. Thin Jurassic deposits of the Ellis Group and Morrison Formation accumulated in a backbulge depozone. A regional unconformity and/or paleosol zone separates the Morrison from Early Cretaceous foredeep deposits of the Kootenai Formation. The model presented here is consistent with regional deformation events registered in hinterland regions, and challenges previous interpretations of a strongly diachronous onset of Cordilleran foreland basin deposition from northwestern Montana to southern Canada.