A non-thermal ultrafast magneto-optical response of antiferromagnetic NiO to irradiation with an intense optical pulse was observed. With temporally overlapping pump and probe pulses, the circularly polarized pump pulse induced rotation of the linear polarization of the probe pulse along with two-photon absorption and cross-phase modulation. The sign of the rotation of the linear polarization reversed with the reversal of the pump helicity. This non-resonant phenomenon is described as a third-order nonlinear optical effect and is ascribed to an inverse Faraday effect: photoinduced magnetization. (C) 2010 Optical Society of America