Frequency-selective or even frequency-tunable terahertz (THz) photodevices are critical components for many technological applications that require nanoscale manipulation, control, and confinement of light. Within this context, gate-tunable phototransistors based on plasmonic resonances are often regarded as the most promising devices for the frequency-selective detection of THz radiation. The exploitation of constructive interference of plasma waves in such detectors promises not only frequency selectivity but also a pronounced sensitivity enhancement at target frequencies. However, clear signatures of plasmon-assisted resonances in THz detectors have been revealed only at cryogenic temperatures so far and remain unobserved at application-relevant room-temperature conditions. In this work, we demonstrate the sought-after room-temperature resonant detection of THz radiation in short-channel gated photodetectors made from high-quality single-layer graphene. The survival of this intriguing resonant regime at room temperature ultimately relies on the weak intrinsic electron–phonon scattering in monolayer graphene, which avoids the damping of the plasma oscillations present in the device channel.