We present a micro-mechanical terahertz (THz) detector fabricated on a silicon on insulator substrate and operating at room-temperature. The device is based on a U-shaped cantilever of micrometric size, on top of which two aluminum half-wave dipole antennas are deposited. This produces an absorption extending over the [Formula: see text] THz frequency range. Due to the different thermal expansion coefficients of silicon and aluminum, the absorbed radiation induces a deformation of the cantilever, which is read out optically using a 1.5 μm laser diode. By illuminating the detector with an amplitude modulated, 2.5 THz quantum cascade laser, we obtain, at room-temperature and atmospheric pressure, a responsivity of [Formula: see text] for the fundamental mechanical bending mode of the cantilever. This yields noise-equivalent-power of [Formula: see text] at 2.5 THz. Finally, the low mechanical quality factor of the mode grants a broad frequency response of approximately 150 kHz bandwidth, with a thermal response time of ∼ 2.5 μs.