Room-temperature operation is essential for any optoelectronics technology that aims to provide low-cost, compact systems for widespread applications. A recent technological advance in this direction is bolometric detection for thermal imaging(1), which has achieved relatively high sensitivity and video rates (about 60 hertz) at room temperature. However, owing to thermally induced dark current, room-temperature operation is still a great challenge for semiconductor photodetectors targeting the wavelength band between 8 and 12 micrometres(2), and all relevant applications, such as imaging, environmental remote sensing and laser-based free-space communication(3-5), have been realized at low temperatures. For these devices, high sensitivity and high speed have never been compatible with high-temperature operation(6,7). Here we show that a long-wavelength (nine micrometres) infrared quantum-well photodetector(8) fabricated from a metamaterial made of sub-wavelength metallic resonators(9-12) exhibits strongly enhanced performance with respect to the state of the art up to room temperature. This occurs because the photonic collection area of each resonator is much larger than its electrical area, thus substantially reducing the dark current of the device(13). Furthermore, we show that our photonic architecture overcomes intrinsic limitations of the material, such as the drop of the electronic drift velocity with temperature(14,15), which constrains conventional geometries at cryogenic operation(6). Finally, the reduced physical area of the device and its increased responsivity allow us to take advantage of the intrinsic high-frequency response of the quantum detector(7) at room temperature. By mixing the frequencies of two quantum-cascade lasers(16) on the detector, which acts as a heterodyne receiver, we have measured a high-frequency signal, above four gigahertz (GHz). Therefore, these wide-band uncooled detectors could benefit technologies such as high-speed (gigabits per second) multichannel coherent data transfer(17) and high-precision molecular spectroscopy(18).