We present an ultrafast graphene-based detector, working in the THz range at room temperature. A logarithmic-periodic antenna is coupled to a graphene flake that is produced by exfoliation on SiO2. The detector was characterized with the free-electron laser FELBE for wavelengths from 8 mu m to 220 mu m. The detector rise time is 50 ps in the wavelength range from 30 mu m to 220 mu m. Autocorrelation measurements exploiting the nonlinear photocurrent response at high intensities reveal an intrinsic response time below 10 ps. This detector has a high potential for characterizing temporal overlaps, e. g., in two-color pump-probe experiments. (C) 2013 AIP Publishing LLC.