We report the experimental demonstration of a multiplier phase detector implemented with a single top-gated graphene transistor. Ambipolar current conduction in graphene transistors enables simplification of the design of the multiplier phase detector and reduces its complexity in comparison to phase detectors based on conventional unipolar transistors. Fabrication of top-gated graphene transistors is essential to achieve the higher gain necessary to demonstrate phase detection. We report a phase detector gain of −7 mV/rad in this letter. An analysis of key technological parameters of the graphene transistor, including series resistance, top-gate insulator thickness, and output resistance, indicates that the phase detector gain can be improved by as much as two orders of magnitude.