We studied the stress-dependent piezoresistive behavior of tunneling-percolation systems. Starting from percolation-like power law of resistivity, a model relating piezoresistive stress coefficient to the applied stress has been developed by considering the stress-induced changes in the critical exponent and that in the concentration of conducting phases. It is found that the coefficient exhibits reverse and logarithmic dependence on the applied stress in the different stress ranges. We show that the experimental data, obtained from carbon-reinforced polymer composites and polymer-derived ceramics, follow the theoretical prediction of the model very well, indicating that the stress effects described by the model indeed exist in tunneling-percolation systems.