Biomimetic, stimuli-responsive nanocomposites were made using either poly(styrene-co-butadiene) (SBR) or polybutadiene (PBD) as the hydrophobic, low-modulus matrix and hydrophilic cellulose whiskers isolated from tunicates (TW) as the high-modulus filler. These materials were prepared using a template approach, which involves the formation of a percolating TW network and filling this template with either of the matrix polymers. Dynamic mechanical analysis (DMA) studies of the dry nanocomposite films reveal that the incorporation of TWs into the rubbery polymers increases the tensile storage modulus E significantly. The reinforcement is attributed to the formation of a three-dimensional TW network within the SBR and PBD matrices. The incorporation of the TWs did not affect the main relaxation temperature of the matrix SBR polymer, suggesting weak nanofiller-polymer interactions. Thus, the reinforcement is primarily on account of the nanofiller-nanofiller interactions, which involve hydrogen bonding. Interestingly, submersion of these hydrophobic matrix nanocomposites in water results in dramatic softening, consistent with disengagement of the TW network as a consequence of competitive hydrogen bonding with water. The kinetics of the modulus change and the amount of water uptake were shown to depend on the TW content. Given the hydrophobic nature of the matrices, it is proposed that the TWs create a percolating network of hydrophilic channels within the hydrophobic SBR and PBD matrices.