The Cenozoic sedimentary succession of Bangladesh provides an archive of Himalayan erosion. However, its potential as an archive is currently hampered by a poor lithostratigaphic framework with limited age control. We focus on the Hatia Trough of the Bengal Basin and the adjacent fold belt of the Chittagong Hill Tracts which forms the outermost part of the west-propagating Indo-Burmese wedge. We present a basin-wide seismic stratigraphic framework for the Neogene rocks, calibrated by biostratigraphy, which divides the succession into three seismically distinct and regionally correlatable Megasequences (MS). MS1 extends to NN15-NN16 (ca. 2.5–3.9 Ma), MS2 to NN19-NN20 (ca. 0.4–1.9 Ma) and MS3 to present day. Our seismic mapping, thermochronological analyses of detrital mineral grains, isotopic analyses of bulk rock, heavy mineral and petrographic data, show that the Neogene rocks of the Hatia Trough and Chittagong Hill Tracts are predominantly Himalayan-derived, with a subordinate arc-derived input possibly from the Paleogene IndoBurman Ranges as well as the Trans-Himalaya. Our seismic data allow us to concur with previous work that suggests folding of the outer part of the west-propagating wedge only commenced recently, within the last few million years. We suggest that it could have been the westward encroachment and final abutment of the Chittagong Hill Tracts fold belt onto the already-uplifted Shillong Plateau that caused diversion of the palaeo-Brahmaputra to the west of the plateau as the north-east drainage route closed.