Using 125 ps laser pulses, we observe three-photon absorption in room temperature InAs at a wavelength of 9.54 μm. This effect is readily identified by temperature-tuning the semiconductor bandgap through the three-photon absorption edge. A three-photon absorption coefficient of K3 = 1 ± 0.6 × 10−3 cm3 MW−2 is extracted from non-linear absorption data obtained with an open-aperture Z-scan. Time-resolved measurement at high irradiance reveals an increase of absorption due to hot carriers. We also present an autocorrelation measurement of our CO2 laser pulse that shows two complete optical free-induction-decay cycles.