To better define the base of the Brahmaputra River paleovalley, we analyzed an extensive borehole data set from the subaerial Bengal delta and a 255-km-long multichannel seismic survey along the modern river. The data reveal that the paleovalley floor is defined by a gravel unit containing boulder-sized clasts up to 30 cm in diameter, deposited after ca. 30 ka but before ca. 9 ka. Paleohydrology during that time and the previous glacial maximum was characterized by a weak monsoon and reduced river discharge, both of which are inconsistent with large valley formation. However, our work indicates that glacial-lake outburst floods sourced from the Tibetan reaches of the Brahmaputra were routed through the lowstand valley, producing megaflood-scale discharge capable of transporting gravel and cannibalizing the valley margins. The timing of these glacial-lake outburst flood–driven discharge events was coincident with valley development and explains the anomalously large width of the valley and basal gravel surface. Despite the underfit scale of Brahmaputra discharge following the last glacial period, a strengthening monsoon and high sediment discharge in the early Holocene subsequently contributed to the efficient infilling of the massive paleovalley by the mid-Holocene. In a sequence stratigraphic context, this work provides an example of a major unconformity that developed late in the eustatic cycle (i.e., during early transgression rather than an earlier, protracted response to sea-level lowering) and in response to a perturbation originating in the catchment instead of changing accommodation in the basin. As such, it represents a geologically instantaneous time surface that can be used as a marker for stratigraphic correlation but one that is not in phase with eustatic sea-level fall.