Graphene mid-IR photodetectors are a promisingchoice for on-chip spectroscopy due to their broadbandphoto-response. However, the low efficiency of couplinglight to single-layer graphene hinders the detectorresponsivity. Here, we demonstrate a plasmon-enhancedgraphene-based photothermoelectric detector operating at4.2 μm wavelength in the mid-infrared. Integratingmetallic resonators with the graphene detector tripled itsresponsivity compared to a pure graphene device,attributed to enhanced graphene-light interaction. Ourminiaturized detector is bias-free and has a fast frequencyresponse of 25.6 kHz. Our detector was implemented in a150 mm silicon-on-insulator (SOI) platform, showing itspotential for on-chip integration and high-volumeproduction.