The volume and grain-size of sediment supplied from catchments fundamentally control basin stratigraphy. Despite this, few studies have constrained sediment budgets and grain-size exported into an active rift and compared this to the characteristics of depositional stratigraphy. Here, we use the Gulf of Corinth as a natural laboratory to quantify the controls on sediment export within an active rift. We measured the hydraulic geometries, surface grain-sizes of channel bars and full-weighted grain-size distributions of river sediment at the mouths of multiple catchments draining the rift, which constitute 83% the Gulf’s drainage area. Results show that grain-size increases westward along the southern coast of the Gulf and that median and coarse-fraction of the sieved grain-size distribution are primarily controlled by bedrock lithology, with late Quaternary uplift rates exerting a secondary control. We also demonstrate that the median and coarse-fraction of the grain-size distribution are predominantly transported in bedload; however, typical sand-grade particles are transported as suspended load at bankfull conditions, suggesting disparate source-to-sink transit timescales for sand and gravel. Using this data, we derive both a Holocene sediment budget and a grain-size specific bedload discharged into the Gulf of Corinth using the grain-size measurements and previously published estimates of sediment fluxes and volumes. Finally, we demonstrate that at the scale of individual fault blocks, these sedimentary signals fundamentally control the nature of basin deposition, including the characteristics and architectures of hangingwall Gilbert deltas. Our results therefore highlight the importance of linking geomorphic and stratigraphic approaches in areas of active extension.