A giant nonlinear optical response has been observed for nanoporous layers of titanium dioxide (anatase) under picosecond laser excitation with photon energy below the gap. At excitation intensity of 10 MW/cm2 the nonlinear refractive index variation at the wavelength of 1064 nm corresponds to χ(3)=2×10-5 esu which is six orders of magnitude higher than the respective value of bulk TiO2. This effect is explained by resonant excitation of electronic states of defects at the developed surface of anatase nanoparticles.