Copper niobate with CuNb3O8 composition was prepared via one-step solid state reaction (750 degrees C, 12 h). Powder X-ray diffraction (XRD) confirmed the formation of parental CuNb2O6 phase with traces of CuNb3O8 on initial sintering (till 9 h), which was then transformed into pure CuNb3O8 on higher sintering (12 h). Field emission scanning electron microscope studies (FESEM) illustrates the formation of truncated cubes due to unification of layered structure upon prolonged heating. Third-order nonlinear optical properties of truncated CuNb3O8 was studied by Z-scan technique using Ti: Sapphire laser (800 nm, 150 fs, 80 MHz). Pure CuNb3O8 possessed strong nonlinear absorption (two-photon absorption coefficient of 5.3 x 10(-10) m/W) and nonlinear refractive index n(2) (2.1 x 10(-16) m(2)/W) compared to mixed copper niobate (CuNb2O6 center dot CuNb3O8). The third-order nonlinear optical susceptibility of CuNb3O8 was similar to 10(-11) esu and was higher than the known nonlinear optical (NLO) systems such as lithium niobate and sodium niobate measured at similar pulse duration. Enhancement in nonlinearity arises due to the influence of morphology and here truncated cubes with the platonic solids offer high polarizability as the charges are concentrated in the edges of the system. The truncated cube CuNb3O8 exhibited strong optical limiting action with a low limiting threshold (34.6 mu J/cm(2)), thus making it appealing for ultrafast optical limiter devices towards photosensitive component protection from Infrared (IR) laser damage.