The formation of an energy-barrier at a metal/molecular semiconductor junction is a universal phenomenon which limits the performance of many molecular semiconductor-based electronic devices, from field-effect transistors to light-emitting diodes. In general, a specific metal/molecular semiconductor combination of materials leads to a fixed energy-barrier. However, in this work, a graphene/C-60 vertical field-effect transistor is presented in which control of the interfacial energy-barrier is demonstrated, such that the junction switches from a highly rectifying diode at negative gate voltages to a highly conductive nonrectifying behavior at positive gate voltages and at room temperature. From the experimental data, an energy-barrier modulation of up to 660 meV, a transconductance of up to five orders of magnitude, and a gate-modulated photocurrent are extracted. The ability to tune the graphene/molecular semiconductor energy-barrier provides a promising route toward novel, high performance molecular devices.