Stimuli-responsive materials are desired for a wide range of applications. Here, we report the design and fabrication of all-organic, stimuli-responsive polymer composites using electrospun nanofibers as the filler. The incorporation of 4 wt % of filler into the polymer matrix increased the tensile storage modulus by 2 orders of magnitude. Upon exposure to water, the filler fibers plasticize and no longer provide mechanical reinforcement. The tensile storage modulus subsequently diminishes 2 orders of magnitude to the value of the neat matrix polymer.