The dipole polarizabilities, second-order polarizabilities, and origin of second-order nonlinear optical (NLO) properties of trisorganotin-substituted, ss-Keggin polyoxotungstate [XW9O37(SnR)(3)]((11-n)-) (X = P, Si, Ge, R = Ph; X = Si, R = PhNO2, PhCtCPh) have been investigated by using time-dependent density functional response theory. This class of organic - inorganic hybrid complexes possesses a remarkably large molecular second-order NLO response, especially for [SiW9O37(SnPhCtCPh)(3)](7-) (system 5) with the static second-order polarizability (ss vec) computed to be 1569.66 x 10(-30) esu. Thus, these complexes have the possibility to be excellent second-order nonlinear optical materials. Analysis of the major contributions to the, vec value suggests that the charge transfer from the hetero-polyanion to the organic segment along the z-axis plays the key role in the NLO response of [XW9O37( SnR)(3)]((11-n)-). The computed ss vec values increase as a heavy central heteroatom changes in the order Ge > Si > P. Furthermore, nitro substitution on the aryl segment and the lengthening of organostannic pi-conjugation are more important in enhancing the optical nonlinearity, especially for the latter factor. The present investigation provides important insight into the origin of the NLO properties of trisorganotin-substituted heteropolyoxotungstate.