The fact that the hydrostatic pressure distribution is implicit in the definition of the Shields parameter does not seem to be widely recognized. This hypothesis is accurate in the case of uniform and rectilinear flow, but fails where local non-hydrostatic pressure distributions can affect the mobility of sediment and hence the bedload transport rate. The aim of the present work is to investigate the effect of a non-hydrostatic pressure distribution on sediment transport and derive a generalized empirical formula to evaluate the bedload transport rate. Both upward and downward seepage flows through a sediment bed were used to create a nonhydrostatic component of pressure affecting the particles at the bed surface. Experiments were carried out in a sand-recirculating flume with a steady main flow, where we created a groundwater flow in a short reach of the flume. The experimental results showed a clear effect of the groundwater flow, and hence of a non-hydrostatic pressures distribution, on sediment transport.