Controlling, detecting and generating propagating plasmons by all-electrical means is at the heart of on-chip nano-optical processing1, 2, 3. Graphene carries long-lived plasmons that are extremely confined and controllable by electrostatic fields4, 5, 6, 7; however, electrical detection of propagating plasmons in graphene has not yet been realized. Here, we present an all-graphene mid-infrared plasmon detector operating at room temperature, where a single graphene sheet serves simultaneously as the plasmonic medium and detector. Rather than achieving detection via added optoelectronic materials, as is typically done in other plasmonic systems8, 9, 10, 11, 12, 13, 14, 15, our device converts the natural decay product of the plasmon—electronic heat—directly into a voltage through the thermoelectric effect16, 17. We employ two local gates to fully tune the thermoelectric and plasmonic behaviour of the graphene. High-resolution real-space photocurrent maps are used to investigate the plasmon propagation and interference, decay, thermal diffusion, and thermoelectric generation.